I just don't think that Angel would have been able to fit back into Buffy's show on a full time basis by Season 5, and that's not just because I support the Spuffy relationship. Angel had been off on his own for 2 years, running his own crew, not having to answer to anyone. Moving back to Sunnydale, aside from having to admit defeat out in the big, bad world, would have wound up putting him in conflict with Buffy eventually, simply because you can't have 2 bosses on the same job. Remember Angel telling Buffy to not come to "his town" and try to give orders? Well, I believe that the same thing would have eventually happened in Sunnydale, only in that case, it was Buffy's town, no longer his. (Actually, Sunnydale always was Buffy's, with Angel playing, at best, 3rd in command behind Buffy and Giles. That's the part that I just don't think that Angel could accept again)
And let us not forget that, by Season 5, Buffy and Riley were fairly stable in their relationship. Doomed to failure in the long run, certainly, but not at the beginning of Season 5. Angel would have only caused confusion and conflict there. I, like just about everyone else here, honestly feel that Buffy truly loved Angel, but I just can't see her as the type of girl who would dump her current boyfriend immediately after her old lover comes back to town. Eventually, almost certainly, she would have wound up doing it, but that would have led to an even more messy situation than Buffy's split with Riley actually was. It might have even come down to a conflict more violent that that one little scuffle that Riley and Angel did have, and, remember, Angel didn't have a chip to hold him back. If Riley provoked him enough, which he almost certainly would have done, Angel would have killed him. He might regret it after it happened, but, if given no other choice, he would have done it, Riley would have been just as dead, regardless.
And speaking of chips, how would the situation evolved between Angel and Spike? By Season 5, Buffy had pretty much accepted him as a member of her team, or at least an asset to her in her fight. How would Angel have adjusted to that? Would Buffy have defended Spike then as strongly as she did in Season 7? Probably not, but I think that she would still see Spike as relatively harmless to the human population of Sunnydale. On the other hand, there were two other Summers women who liked Spike and considered him a friend. I don't think that Joyce or Dawn would have been too thrilled to have Angel hurting him, and I don't think that it would have been a case of Spike hiding behind Joyce's skirt tails. Joyce would have known immediately what was going on. And what about Dawn? I wouldn't be surprised a bit to have seen her going after Angel with her mother's fire axe if she thought that he was trying to hurt her friend. Then there's Spike himself. Maybe he didn't have the self confidence at that point to realize that he could beat Angel, or at least hurt him badly, but he would definitely have fought, if only in self defense.
So, here's Buffy. Her old boyfriend has just come back to town, putting a strain on her relationship with boyfriend #2, Spike and Angel are going at each other like a pair of pit bulls, with her mother and sister are on Spike's side. Xander is standing off in a corner laughing his a** off, not caring which vampire wins, just as long as there is plenty of pain received by both. Anya is mad at him because he's watching Spike and Angel fight when he should be giving her multiple orgasms. Giles has polished the lenses completely out of his glasses. Willow and Tara are off somewhere giggling and cuddling, completely oblivious to the entire situation.
And then, just to make the Slayer's day complete, up pops Glory with sinister plans involving what she and everyone else believe is her spoiled, bratty, but still beloved, baby sister. Then there's the Buffy Bot, the Knights of Byzantium, and we are beginning to think that Ben might be somehow involved with Glory.
Now pile most, if not all, of that on top of Joyce dying, Hank being somewhere on the other side of the world, and Boyfriend #2, who, assuming that Angel hasn't actually twisted his head off, gives Buffy an ultimatum: "Give me a bigger part of your life, or I'm gonna run away to South America and marry the first Peace Corps worker that I can find. Hopefully, it'll be a woman, but with a name like Sam..."
Now, granted, the events of Season 5 didn't occur in precisely the order that I have listed them here, but they did all happen, and, assuming that Angel's returning to Sunnydale wouldn't have resulted in a completely different story line, by the time of Season 5's final episode, Buffy would have been begging Glory to let her jump off of that tower, just for the peace and quiet of the grave.
