I think he did but that's not why she came back. She intiated things before he did that but it's a number of factors what stopped her was seeing concrete proof that he would go behind her back and collaborate in schemes that would result in deaths it didn't matter how high up he was on the food chain or how much he knew - he didn't care even if it was as simple as not caring enough to question that was still morally compromised enough to be too toxic and he was literally incapable of understanding that.
As to the question I don't think Buffy was depressed because she sacrificed herself and was ressurected but she did the former because she was already becoming depressed. Think about it - beverement, relationship break up, work stress, threat to last close relative - all triggers in their own combine them and add money worries and a dead end job. She was seeking an escape and with a lack of help and therapy she got sucked in to what was available and it didn't help everyone around her had their own issues. In a normal state of mind Buffy would have had the ability to set healthy boundaries between her and Spike and maybe he would have made the choice he did without the trauma the damage they did to each other but she definately wouldn't have had the affair if her mother hadn't died.
You make some great points. In terms of the cornering her before their "relationship", there are several instances wehre she goes away to be alone, and he always seeks her out even when she wants some solitude. He makes a point of being there whenever she is upset (not quite as sweet as it sounds, given the motive), in order to establish that when she's not doing well, she should come to him (as seen in "Life Serial). I find that a little icky...Can't really pin point why, (I don't think he starts verbally maipulating her until they are "together") but even so, it doesn't quite sit right.
I think Buffy was extremeyl stressed, yes, but Buffy
did not want to die in "The Gift". In my personal opinion, that is the reason the gift is so meaningful and important for Buffy's character. It wasn't two birds with one stone, she wasn't saving Dawn
and getting a free ride outta dodge. She wanted to live, but she loved Dawn and her friends/family (heck, even the randos she's never met but lives to protect anyways) she would lay down her own life for them.
"For what greater love is there, than to lay down one's life for one's friends"?
You are right, she was not depressed because she sacrificed herself, she was depressed because, as she said, she was finally fullfilled, and now, suddenly, she is back playing Sisyphus in Sunnydale. Not only that, but because she's been gone, her whole life is off track. Add to that, she's never had the opportunity to learn how to be an adult in society (though, of course, her attitude and psychology have matured) and thus doesn't know anything about getting a job, paying bills, yada yada yada. She never had the chance. One thing I really don't like about season 6 is Buffy's friends (Except Tara. Tara really proves herself this season.). They pull her out of Heaven (Unknowingly, sure), having used all her money and moved into her house. Then they have the audacity to
tell her she has to get a job. I never understood why Willow and Tara couldn't have gotten a job. Plenty of people have Jobs
and go to school. If Tara, Willow,
and Buffy all got part time jobs, they would probably be much better off than Willow and Tara doing whatever, eating all Buffy's food as Buffy struggles to balance being the slayer, keeping a job, and just generally stay afloat. For get about going back to school, she's drowning in debt and she has no time.
And then Giles come's back. yay, right? No. He comes back, and as soon as Buffy says, "Thank you so much for being here. I've been suffering, and I really needed someone to help me feel stable" (Paraphrased) he decides he has to leave, because this 20 year old who just lost her mom and is trying to support herself, her sister and her two friends, and struggles with a dangerous, unforgiving job, and was never taught how to live as an adult apparently needs to blindly figure out how to support heself, RIGHT NOW.
Unfortunately, it really isn't a stretch fro Buffy to become depressed. The world is almost literally against her.
Bored now.