"Or how she let me literally cry on her shoulder, her wishing she could do something to save my marriage, while she was sleeping with him."
As for this, your ASSumption is that she was lying, being intentionally evil, maybe secretly enjoying her duplicity.
I doubt it.
What else could have been going on? Maybe she was in denial, lying to herself. Surely you can understand denial of the obvious? Again, what lies and wishful thinking and denial were included in what XH and GF were telling themselves and each other... This will never happen again, this is just temporary while we get our lives together, just a temporary crutch, we'll never let it happen again, we both want each other's marriage to work, we don't want to break up the families, maybe we can help each other have better marriages, blah blah blah. Affairs aren't easy for people in long committed Ms. Their thinking would have been crazy and all over the place.
Moreover, she might have had to let you put your head on her shoulder because she literally couldn't bear to look at you and thereby be confronted by herself. Avoidance.
She may well also have been sincere, wishing she could save your M, but believing it to be dead before she came on the scene. Affair partners generally don't believe they caused a marriage to fail, and generally I think they are right. She may have been sorry about how messed up all your lives were, wishing they could magically be how they were supposed to be.
So, given her likely belief that she did not destroy your M, (warning weird analogy) to her it may have seemed like sincerely saying you are sorry that someone died while still being willing to take an organ of the deceased to save your life.
While, to you, it seems like lying about being sorry that someone is dying while you bleed them out so you'll be able to help yourself to an organ upon their death.
Only the second version is really evil. But, that is the kind of story you tell yourself about GF and her comforting you.
Again, who knows what was going on in that scenario. But if you extend some compassion and detach, maybe you can see that the non-evil version is probably more likely...
