Why would Willow "never" ask Faith to help in Season 5, then ask, or accept Faith's offer, I can't remember exactly which happened, in Season 7? What changed during those 2 years? Also, I didn't get the feeling that there was any hatred coming from Willow during their trip alone back to Sunnydale. They weren't acting like long lost friends, but they didn't seem to be any feelings of hate or distrust, either. Are you saying that Faith's helping Angel's team somehow convinced Willow that Faith had changed that much, and that her intentions were, in fact, honorable?
In S3 we saw that the first person to 'give up' on Faith was Willow. She called her a murderer and was pretty much ready to say good riddance to her as early on as 'Consequences', when Faith was genuinely saveable. At least until Wesely's interfereance.
Willow is also the most vocal opponent to Faith in S4- saying that Buffy should 'beat the crap' out of her from the get-go, openly calling Faith a slut-bomb in Tara's presence- that sort of thing.
That's fair enough, Faith's a bad guy, but this is
Willow we're talking about, the woman who in the past has shown true compassion for murderers (James in IOHEFY, and Pete/Debbie in B&TB) where others (Buffy and Xander) have seen plain evil.
As to why I suspect it's that Willow (or a part of her at any rate) resented Faith not because of what she did (apart from the sleeping with Xander bit) but because she had
power- power that she hadn't earned but was just handed for no real reason (see Dark Willow's speech to Buffy in S6) Part of Willow is also very jealous of Buffy' s power. 'I'm not you're sidekick! and so forth.
By S7 though Willow had changed. She, like Faith had known corruption due to power and had taken lives (and tried to kill her friends) because of it. She was therefore much more sympathetic to Faith's fall and subsequent path to atonement. I think they could relate to each other much better than they could before.
It would've been nice to see some sort of conversation about this but we can infer stuff (Willow sympathetically asking about jail, feeling genuine guilt for not warning Faith that she might be in danger, etc.) and see the change in Willow's attitude.
And, like you say, Faith had just helped out Angel and Co. which garnered a little trust (not really with Buffy) but it was Willow's experiences in S6 that made it so she could see where Faith was coming from.
And, after Season 7, Faith obviously didn't return to prison, and she was, apparently, still a wanted fugitive in Season 8, since part of her bargain with Giles was that he would somehow clear her record if she agreed to impersonate the English girl. Once more, what do you see as having changed between Season 5 and Season 7? Possibly Faith realizing that, with all the new Slayers popping up all over the world, she was genuinely doing more good free than in jail?
I was thinking more Angel's speech to Faith in 'Opheus' telling her not to give in and die, to keep fighting, and that her time was never up, that she still had more to do, had Faith thinking about not going back to prison after S7.
Add to that her couple of near-death experiences, getting drugged, blown up, nearly drowning and stuff. Maybe Faith figured that she really could do much more good out in the real world, where slayers like her were in need of guidance.
If she hadn't had all of that then maybe (and this is pure conjecture) she would've still thought she deserved prison even after helping defeat Glory.