Also, during those few moments when the conversation isn't directly about our son, I feel like she's trying to instigate something. Like she wants to push my buttons and cause me to blow up. Could this be an attempt to validate her choice, to have me get angry and say something nasty that would illustrate that I really am the reason she's unhappy? I haven't fallen for the bait, and even do a pretty good job of validating.
Yes, I think she does it to justify her leaving what made her unhappy (in her viewpoint). When she begins to feel a little doubt, she'll try to engage you into an argument to reinstate to her that you're to blame for all that's wrong in her life.
I suggest you practice agreeing with her........whenever you can. Even making comments like, "You may be right", could stop an impending argument. And, with the track record of both of you, trying to get your point across to her is usless right now. I think she needs a lot of validation. Don't kiss her a$$, just listen and validate, if you can only nod your head while she's talking. Looking directly in her eyes while she talks, is important to women. Don't be creepy, trying to follow her around and get in front of her to see her eyes, just look at her, so she'll know you hear her. if she looks away, then wait for her to look back at you. She doesn't want emotional pressure. Don't waste your breath trying to persuad her to your way of thinking about her or the MR. It will only verify her feelings that you are her problem.
If she is approaching her unhappy life by eliminating possible sources, it will take her time to realize the true source.
Regarding her job, is she caring for children in diapers? Is this PreK or more like child care? I could understand her being certified as a PreK teacher, but feeling as if she can't fulfill what she has been trained to do. This could be a realistic source of discontent. It could also feed her low self esteem. Has she tried to get a position with three & four years olds?
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!