OT: Obi Wan explaining Vader to Luke in ROTJ

First, let me preface by saying that every Star Wars film written and directed by George Lucas is filled with clumsy writing. Even the best one (Empire) has clumsy writing (just less of it). 

Second, you are also correct in that Star Wars was conceived as a stand alone film, not a trilogy. So it was written without sequels in mind. That being said, the ad hoc way around this that he concocted for Empire is actually pretty good. This is one area where I feel like it was actually strong writing that makes the plot deeper than it would have been had Vader just been some evil guy who murdered Luke's father. That would have been so cliched and cartoonish. Writing it so that Vader is Luke's father adds depth and dimension to both characters.

The explanation that Obi Wan gives also fits in line with his (and Yoda's) desire to keep Luke (and Leah) hidden from the Sith for as long as possible. Luke knowing that Vader was his father in the original film would have given him an emotional connection to Vader that would have allowed Vader to detect him through the force much sooner. It was only through Emperor Palpatine in Empire that Vader became aware of Luke's lineage. Had he found out earlier in the series, the entire plot of the original film would have ended up differently. He would have tried to capture Luke and turn him to the dark side instead of trying to destroy him. Another reason to keep Luke ignorant: Obi Wan knew that if Luke was aware that Vader was his father that would make it more difficult for Luke to do what "needed to be done" when the time was right. From Obi Wan's perspective, Vader was pure evil at this point, and only Luke (the last Jedi) could destroy him. If Luke failed, all hope was lost. He couldn't risk allowing Luke to be emotionally compromised or have any hesitation about what he needed to do.

Also, Luke finding out from Vader that he was his father instead of finding out from Obi Wan changes Luke's perspective on both Obi Wan (and Yoda). He realizes there are more nuances to the force than simply "good" and "evil". Knowing that Vader was his father enabled him to feel some good in him that neither Yoda nor Obi Wan could feel, while simultaneously realizes that what he previously viewed as the epitome of "goodness" was capable of lying to him and deceiving him (even if it was for his own good).

The latter.

Lucas can claim what he likes, but it's pretty clear he didn't envisage Star Wars sequels when he wrote the original. It's also extremely clear that he didn't intend Vader to be as important as he became, didn't intend him to be Luke's father, and certainly never intended Leia to be his sister.