Watcher Guardianship

I have my own fan theory - complete conjecture - that the reason Buffy was overlooked, was that she was never technically on their radar to begin with. She was never a conventional "Potential Slayer," but rather her cousin, Celia - whom we learned about in "Killed by Death," who died when Buffy was 8; and with her when she died - was actually a Potential; and she died at the exact same instant the active Slayer at that time died. As a result, the Slayer powers connected with Buffy, a young girl with raw, untapped Potential, and closest to Celia just as her power were about to take hold; all of which acted as sort of a lightning rod for the Slayer powers.

The bigger twist on this would be that Celia's Watcher was none other than Hank Summers, Buffy's father. It was because he was assigned to be Celia's Watcher that he ended up in Joyce's life - Joyce being Celia's biological aunt. Hank's marriage was only partly a cover, as he truly did fall in love with Joyce. And he naturally cared very much about Celia, as most Watchers do with their charge; and her death was very difficult for him.
The death of the previous Slayer would of course be a big deal for the Watcher's Council, they would need to identify who the new Slayer was right away, so they could receive all the necessary resources and attention. But by all accounts none of the Potentials that they had been training around the globe appeared to have been active; it was unprecedented. Ultimately their only conclusion was a rogue Potential; someone they managed to overlook for whatever reason, who despite all the odds that favored one of the Potentials in their care, had been called as the new Slayer. It had never happened before, but many knew that it was bound to happen eventually. The Council began employing all manner of mystical tools to search for the Slayer; probably similar to the techniques shown in season 7 for identifying Potentials. In fact it may have been the exact same process, the only problem being that the world is large, there's a lot of people in it; and the spell that finds the Slayer also identifies Potentials, so it's not so much like finding a needle in a hay stack, is it is looking for a specific needle in an immense hay stack, riddled with other needles.

Meanwhile, Hank Summers, dutiful Watcher who's lost his charge, is helping with the search. He's reviewing the case information, studying when the last Slayer died and search for any correlating events around the same time or shortly after that might help narrow the search; and something does stand out to him - the last Slayer's time of death is the same time Celia died. It's not even Hank who realizes this, not entirely and not first. Hank is working late at home when lil' Buffy has come down stairs after having a bad dream; she asks what he's doing and he explains it's something for work. She sees the date mentioned in his files and asks if it had something to do with Celia, because that was the day she died. As he's about to dismiss the connection, a flash of insight occurs to him; Celia died the same day the last Slayer died, but surely that's a coincidence. He checks for the estimated time of death of the last Slayer before tucking Buffy back into bed. She gives him a hug that's surprisingly firm for an 8 year old girl and Hank's mind is racing.

Hank digs deeper and finds out the best estimate of the time of the previous Slayer's death and also finds out the time Celia died and as best he can tell they both died at the exact same time; a convergence of circumstances that were astronomical.
But did it mean anything? Could Celia have been the one called and what it would have meant for a Slayer to be called at the moment of their death? He confided these suspicions in a fellow Watcher; a man he trusted more than anyone else in the Council, the man who had been raising the concerns about their methods for identifying Potentials and even how the Council treated an active Slayer - Rupert Giles. Giles was brilliant by any standard, but even he was baffled at what Hank was proposing. It's possible, he tells Hank, that if what he was suggesting did in fact occur, then it was equally possible that because the powers were unable to fully take hold in the new Slayer, Celia, before she died, the ability to call the next Slayer after her may not have had the opportunity taken hold either or be triggered and, "without some sort of anchor to redirect the powers to, the Slayer powers may well be lost. Permanently"

The prospect that the Slayer line may have not only ended, but ended with his charge, because he failed to prevent Celia's death, weighed heavily on Hank; as did Rupert's mention of an anchor. What did he mean by that?
"Someone who might have inherited the powers if they rebounded; a person the powers could find purchase in," Rupert tells him. "Someone who wasn't a Potential?" Hank asks. "Theoretically, at least not in the conventional sense. But it couldn't just be anyone; physical presence would probably be keen, but there would need to be more than that. A Potential isn't some girl who might one day become the Slayer just because a spell says so; the spell used to find new Potentials identifies the traits that the Slayer powers are drawn to. The moral compass, strength of character and most importantly the potential to wield the power of the Slayer judiciously. Someone could have been standing three feet away from her when she died and if they didn't measure up to those exacting standards, I dare say the odds would be very poor that they have received the powers. Why? Do you have someone in mind?"

Hank began putting two and two together and after covertly testing Buffy his suspicions were confirmed; his daughter was the Slayer. He had spent years preparing to become a Watcher, had been a Watcher for his entire adult life, his mother had been a watcher and her mother before her; he knew the risks, he knew the dangers in the world that most weren't aware of and he had faced the loss of his charge, the worst thing any Watcher had to prepare themselves for. But no Watcher ever had to face the prospect of their own daughter being chosen as the Slayer. It was hard enough to train a protege in the form of a Potential, some random girl who may or may not be called; they say not to become too attached to a Potential or to their Slayer if their charge is called, but that's always easier said than done. And never makes losing them any easier. But this is his daughter, his flesh and blood; at best, in a few years he might tell her how he was a Watcher and she might attend the Watcher's academy to eventually become one. Being the Slayer meant she probably wouldn't live long enough to finish school; and even if she did.... God, the Cruciamentum... why does the Council even do that anymore? Why did they ever do it?

Hank knew what he had to do, he was going to hide Buffy from the Council, do everything he could to ensure they never knew she was the Slayer. On some level he knew that his plan would never work long term, but he had to try. He used enchantments to cloak Buffy from any detection spells, the drug cocktail the Council had designed for the Cruciamentum, scaled down and refined for a child's dose; though he was surprised at just how much needed to be administered for Buffy's enhanced strength to be sufficiently suppressed. He began to overindulge her, doing his best to give her anything and everything she ever wanted, to the point that she became rather spoiled. This put a strain on his relationship with Joyce.
Over time it became difficult for Hank to administer drug cocktail as often and slowly her powers, the strength and coordination, began to manifest, though still dulled. Eventually the Council located Buffy and identified her as the Slayer, sending her first Watcher to engage her. After she confronted the vampires at her school, and after the death of Merrick, Buffy felt her only recourse was to tell her parents and confided the truth to her father. He made a plea with the Council; surely there must be a way to remove these powers and transfer them to another Potential, since she was never meant to receive them in the first place. The Council's explored this with tests, disguised as a psychiatric hold; a measure Hank immediately resented, prompting his resignation from the Council and removing Buffy from the facility.

By this point Hank and Joyce's marriage was strained to its breaking point; they were separated and he knew the end of the marriage was near. He saw to it's ultimate end by making up an affair he confessed to Joyce. They divorced and Hank conceded custody of Buffy to Joyce, knowing that while the Council probably wouldn't hesitate taking Buffy from him, they'd have a harder go of it justifying removing Buffy from her mother, who knew nothing of their world. He planted the idea of moving away with Buffy to get a new start after the issue of Buffy setting fire to her school and getting expelled. He'd done some research and identified a quaint, medium sized college town not too far away from LA, that by all outward appearances posed few mystical threats for the Slayer to face - Sunnydale. He even managed to pull some strings to have Rupert Giles assigned as her new Watcher, though Hank was insistent that Buffy must never know the truth about him or that they ever knew each other.
Meanwhile, Hank set out on a quest for peril. He was determined to face any and all danger he could find that might eventually need to be faced by the Slayer; and stop it before it ever got that far.

Right, and in season 7 they showed that there were spells that could help locate Potentials - though it didn't give them any strong indication whether they would ultimately become a Slayer.  So the Council identified and maintained a pool of Potentials, assigning them a Watcher and trained each of them, so that in the event one of them was called they would already be prepared.  Subsequently, most of these girls were killed by the First in early season 7; so of the girls Giles began rounding up to bring to Sunnydale, few of them ever had Watchers or knew that they might one day become the Slayer.

I think what's implied about how all that works and how things in season 7 played out was, the reason why someone like Rona was a Potential, but didn't have a Watcher or know anything about any of this, was that while there were hundreds or thousands of Potentials, they didn't all have an equal likelihood of becoming the next Slayers.  The Council collected those Potentials with the strongest potential, most of whom the First wiped out, leaving Potentials that were still out there who may have been next in line, but were several deviations behind where some of the Council trained Potentials would have ranked in the Slayer line of succession had they not been killed.  Does that call into question whether or not calling all of the Potentials was a good idea?  Maybe, maybe not.  Even if there's a JV tier of Potentials, this would arguably be a case where, the fact that they met the criteria to be in the Potential pool says something about their character as a possible Slayer.  And just because there was some quantifiable aspect about the others the Council trained that might have given them the edge to be called, it doesn't detract from the fact that they were still Potentials.

I don't think so, because my reading of mmexis' comment about some Slayers being surprised and others, like Kendra, knowing all there is to know about being a Slayer and having been removed from her family by the Council at a young is common and a contradiction.  And maybe I'm understanding what they're saying wrong, but I would contend that their interpretation of what we were shown isn't quite correct.
There's no contradiction in someone like Kendra being trained by the Council and sequestered from her family for most of her life, if that's the most common scenario for the vast majority of Slayers; which is arguably the implication if the Watchers Council can locate Potentials and do pluck them from their home lives and train them.  Just because they are Potentials they don't remove and train is also not a contradiction, if the Council has the means of giving themselves the edge by narrowing down the vast number of Potentials to a pool of Potentials with the highest odds of being called at any given time.  That would mean that Buffy, who was called, but not previously trained, was an outlier; because she defied the carefully honed system that allowed the Council to groom 99% of the previous Slayers up to that point.  While Potentials like Rona, who weren't in the know and had no Watcher, would have stayed in the dark and probably never been called had the First not attacked the Slayer line. That act change both the likelihood of her being called the necessity of filling her in on what goes bump in the night.  Rona and Amanda were arguably the King Ralph of possible Future Slayers.