I wonder if some kind of "email friendship" wouldn't work better for you than an "in-person friendship." That way you could truly keep her at a distance, while still conversing with her and being friendly toward her.
I also wonder if during her first eight years with you, she wasn't perhaps really sincere about the poly dynamic, and maybe it was only from 2008 on that she started to develop her cowgirl mindset.
In any case, your best move is to agree to the kind of friendship that *you'd* feel good about, that your *conscience* would feel good about. Forgiveness is great and friendship is fine, as long as it doesn't involve anyone getting sucked into anyone else's whirlpool. Don't befriend her out of a sense of guilt ("I caused her to become this desperate, clinging person"); instead, do it if and only if you *want* to, and feel sure no harm could come of it.
Everyone played a role in what went wrong post-2008. That's now all part of the past, fixed points in the timeline of history that can't be altered. Yes, the here and now is the *only* place where choices can be made, so choose something that will serve the here and now well.
I know that's awfully generalized prose, but it's meant to respect your freedom and judgment in making the choices that affect your life. I'm just an observer (albeit a friendly one).
Love means never having to say, "Put down that meat cleaver!"