I freely admit that this has triggered stuff from experiences with some friends. It's going to be harsher than my normal attempts at tact and understanding.
A dear friend was in a warped unhappy relationship for many long wasted years (my opinion) because in part she kept seeing his beautiful soul and being of light-ness and not his fucked up actions. He had 'potential'.
Years ago, I fell madly in love with a brilliant, charming, funny woman who could not tell the truth to save her life. She persuaded me of all sorts of untruths. I dodged a bullet because she decided she wanted to be friends with me and not date. Still when the lies were revealed, it was deeply damaging to me.
So I say to you, as you consider her potentially beautiful soul filled with light, SO THE FUCK WHAT. Let her go work on her soul. You and White Knight need to run fast the other way.
Beautiful souls do NOT act as she has - manipulating you and your husband against each other, beautifully Idoing her part to erode the trust between you two (yes y'all contributed to that too but let's look at original causes here).
You can't fix her, White Knight can't fix her. By maintaining ANY contact, you invite further drama, crazy, and manipulation into your life. You two cannot handle her - she will spin both of you around until black is white, up is down, and your marriage will explode from the inside with the toxicity.
You two are clearly nice, caring folks. For your sanity and your relationship, RUN. Save yourselves and let her work on her beautiful soul on her own.