Fans Haven’t Guessed Mystery Girl’s Identity Yet

They seriously muddle the whole "powers that be" thing.  I mean, it started off as just an ambiguous term Doyle applied to the unknown source of his visions - he didn't know who or what was responsible, may not have ascribed to any particular faith system, or at least didn't want to make any assumption as to a "right" one; and so he basically described as whomever they might be.  But it quickly morphed into almost a pseudo religion within the series, with everyone talking about "the Powers" and Doyle was his link to them and then Cordelia was; and it became seriously convoluted.
And it's sort of part and parcel with how they throw around the word "champion".  This really stood out in the episode "I Will Remember You," during a recent rewatch; where the Oracles tell Angel that he was "released from his obligation," and I thought, what obligation?  It's not like he was fucking drafted; he (eventually) made a conscious choice to fight the good fight, because it was what he wanted to do (and to impress a girl, but most the right thing part).  And the same goes for virtually every other character in either series; they're faced with the world around them and have choices to make, but their only obligation they're beholden to are the actions they can live with, which is pretty much the character of Angel in a nutshell.  If he had chosen to continue forward in life as human, rather than turning back the clock, there's absolutely no reason he couldn't have still taken up the charge.  He may not be to offer the same degree of fighting abilities as he did as a vampire, but then neither did Gunn, Wesley or Fred, neither did Giles or Dawn.  Surely he'd be more helpful in a fight than Xander.... 

I feel the same way.  Just like the "powers that be" and "champion" they screwed the pooch with the nature of the visions; especially since it was not only a pre-established power, but one that was pretty synonymous with one of their major characters.  That's one of the things I hate about "Birthday," and the idea that any human who had visions died as a result.  The only way I can reconcile it is that it was total BS in order to manipulate Cordelia into following the path laid out for her; because it makes no sense, especially on account of Dru, who's head clearly didn't explode before she was made a vampire and wasn't driven insane by the visions by Angelus' torment.  It all being an elaborate shadow play to get Cordelia's consent also helps explain the other issues of the episode, like Cordelia being drawn to the Hyperion or the address being under the wallpaper in this altered reality; or that after reality resets they don't have to still address the vision she originally had, since the girl was only helped in a reality that no longer exists....

Angel having the visions in the alternate reality was an interesting choice, but still kind of dumb, seeing as how it drove him insane; which worked in complete opposition to Skip's premise that the main problem of Cordelia having the visions was that she was human and that virtually anything else would be fine with them... unless you're a vampire, I guess.  
Not sure how Angel would have met Gunn though, if he was slowly losing his mind with the visions.  Plus, whenever I watch the episode, I have to wonder - how did Doyle pass the visions to Angel?  Every other time they get passed between people in the rest of the show, it's through a kiss; and it was suggested Gru would have gotten them if he and Cordy had sex.....

Regarding the visions themselves, Angelus sensed something a special purity about Dru when Darla first showed her to him; and at first thought that Dru was or would be a saint.  That certainly suggests some type of powerful connection to something special.
It's funny you ponder whether Dru was meant to be an agent for good, as I recently started a thread on the Buffy board expounding on that - IMDB2.freeforums.net/thread/145905/drus-story
Essentially, it occurred to me that out of the four members of the original "fang gang," three of them would have some type of story involving them facing the consequences of their past and feeling remorse; being faced with having a soul, desiring redemption and two of them even become outright champions.  And I couldn't help but wonder if Dru got short changed, being the odd one out, so to speak; especially with her having "the sight"

So in wondering that, and thinking about the whole thing with nature of Angel being a "champion" and the visions, the conclusion I reached is that it would make more sense if the visions weren't so much a mission sent by someone or something with a grand plan, saying, "this person needs to be saved, or that needs to happen to fulfill my intentions," but instead it's the person with the vision's ability to perceive the world around them and know where there's an opportunity to affect positive change.  
If you think about the visions as a message from some higher being or group of higher beings, with an explicit mission, it really becomes questionable then why they direct them to save one person and not someone else; like why don't the visions occur on a more regular basis, why one person deserves saving and someone else who gets hurt or dies didn't - especially in those cases when a vision leads them to someone after it's too late.  On the other hand, I think it would make sense if the "power" is actually coming from the person with the sight; it is after an extra sensory perception; the thought being that Cordelia or Doyle or Dru has a subconscious awareness of events going on, and the possible outcome to those events, but there's so much data there that it would be impossible to consciously process all of it without being driven insane; at least if you're human or even if you're part demon.  We're talking anything short of God with a capital G.  What triggers a vision then is when their subconscious goes through all these permutations and they become aware of events, pending or presently occurring, whereby their intervention has the best chance for a successful and positive outcome.  And so on that subconscious level, they know they can't be everywhere at once; and they know, especially if augmented with the ability to see possible futures and outcomes for subsequent actions, that intervening with certain events might have unintended consequences where it's best to leave it be, even though it means that some people appear to be ignored who might have used their help.  And so that might explain why other large threats don't end up on the radar, like Glory or the First, because on some level they know it'll be taken care of without them; or that they're not in a practical position to be of help, especially if it takes them away from threats, like Jasmine or Holtz or Sahijhan.
The next step after the vision of course is the choice to either follow the vision or ignore it.  This might not seem likely, because who would choose to ignore some of those visions?  But ultimately it is a choice and invariably there are some who would choose to ignore them for a wide variety of reasons - fear being the chief among them, but lacking physical prowess or even some who may simply be indifferent or apathetic, as hard to believe as that might be for many.  But that would ultimately be a truer and more appropriate definition of a champion; someone who has the opportunity to help and chooses to do so, even at great personal risk.  And that would then apply to not just Angel, but the rest of his team.

I kind of touched on this above, with the visions being a choice, and some level of knowledge that by going to help fight a threat elsewhere, they're not present but there are other potential explanations that I've found fun to explore.  Like, maybe Jasmine and the First were in league; or even one in the same - collecting all the Potentials and Slayers in Sunnydale, means they aren't in LA for its big birth into Jasmine.  I feel like there have been some other theories too, but I can't think of them off hand.

I did have one thought with Glory; which is entirely its own tangent and goes more into the territory of fan wank than theory.  
So the idea stems from the events of the AtS episode, "I will Remember Youth" and what might have happened had time been allowed to continue as normal.  Angel, now human, eventually returns to Sunnydale to be with Buffy.  Not long after his return, Buffy learns she's pregnant, as consequence of their romp in LA when Angel being suddenly made them forget about things like protection.  To keep things short(er) I won't get into how season 4 of Buffy might have played out with Angel back in town and Buffy pregnant (plus, not fully settled in my mind).
The way the episodes aired, the timing would be pretty close to where, had Buffy and Angel conceived a child in "I Will Remember You," the child would be born around the start of season 5.  Naturally, as Buffy goes into labor, Willow attempts a spell to help ease her pain; which leads to shenanigans where other members of the Scoobies, including the men, and probably Spike, experience the labor in Buffy's place; with Angel holding their hand and the two expressing their undying love for one another.  The delivery is a success, the spell ends; Buffy lays in the hospital bed with Angel beside her and a nurse presents their son... followed by their daughter.
You see, with Buffy already pregnant, the monks had an opportunity to craft the Key into an infant daughter - someone the Slayer would protect just as much as she would have done for a sister, but without the need to alter or rewrite reality/history.  So they name her Dawn; and they name the boy... Conner.
Conner is Angel and Buffy's son; a child who's descendant of a Slayer and cured vampire, who was meant to be born, only for his conception to be erased when Angel turned back time.  So when Angel attempted to lose his soul by sleeping with Darla, a piece of his soul was already in the ether - that part of him that combined with Buffy's to make this child - and the soul already in Angel acted as a magnet, drawing that piece of him across the folds of time back into being; and through the circumstances of his birth and the wonky spell Willow would have cast, it was able to find a home in the closest surrogate womb.
Of course in the original reality, Conner would have still been a threat to Sahijhan, so Holtz would have eventually come looking for Angel, only this time earlier, and coinciding with Glory searching for the Key; so Sahijhan opens the portal to Quor'toth and with few options, Buffy and Angel, hand-in-hand, each holding a child, leaps through, away from the immediate threats, albeit encounter another.  They return some 13-14 years later, from their perspective; to everyone else, gone only a week or so.