And no, no one led her step by step except the writers who slowly and carefully showed her reemerging trust in Spike step by slow step.

She sent Dawn to Spike's crypt so he could look after her less than an hour after the attempted rape. I think she took a long time to forgive him, but she always had a good idea what he was and wasn't capable of, which is the basis of trusting someone.

As aycheb has often said, Buffy doesn't blame Twilight-the-man for the deaths of her followers. She blames herself, and public opinion turning against Slayers for perfectly valid reasons; not some supervillain in a ludicrous mask. So when Angel shows up to say they everything is happening for a good reason, and there's a plan behind it all - and Buffy has solid, inconvertible evidence that something weird is going on, on account of, you know, she can fly - why shouldn't she believe him? She wants to believe him. Really, really badly.