I think your comment about her not being sure how or why it happened is pretty accurate. There have been at least 3 things that went on during the time of the A (2 non A related and one related to the A) that she just doesn't even remember. I think she recognizes what made her vulnerable, but I really think she doesn't have a clue how she let it go there.
Some people actually have the ability to bring on a dissociative state in NORMAL people. These people are sociopaths of course...some come by it naturally, and some go about LEARNING it, which makes you suitable for prison, IMO.
There are entire websites devoted to teaching predators how to make a person more suggestible (planting ideas in their heads as you say)..how to break down their natural defenses...they WANT a person who isn't "like that" or who would never do anything like that. That's the goal.
I totally agree with you about not getting yourself in a position to invite trouble if you're married, and I have always practiced that. I, too, thought I was in "safe" situation with OM because it was online...and began so seemingly innocently. And it's gradual, how it works. Now I know that it was anything but innocent on his part...it was very calculated. If there was a Hall of Fame for brilliant psychopathic liars, he would be in it.