Thanks for sharing insight, Harri!

I am not sure I see the positive in accepting gifts... or even if I see a neutral outcome in accepting gifts if she and your husband are going to use that to FOG.
I accepted because my daughter needed a desk and because I offend her when I don't take the gifts she offers. Under our current circumstances, declining a gift will appear petulant to them. I also know that accepting may appear selfish to them. "oh, she'll set boundaries but she'll still take my stuff." It feels like a lose/lose/lose situation. So I accepted, knowing that I risk it being held over my head or coming with conditions. Sometimes she takes them back or tells me what I can/cannot do with them.

I'm walking on eggshells, right?
I would do both of the first things you mention: I would bring it up *and* I would chalk it up to his blind denial/enmeshment.
So I did mention it in a simple, non-confrontational texted question. He insists that he didn't share any information. I thanked him for clarifying, and while it is possible that I misinterpreted, he has inappropriately shared info with her before and not understood the boundary when I asked him not to. I don't trust either of them to keep anything confidential. I
can make a choice not to flip out, adjust expectations, inform myself about the disorder, and as you said temper my response.
I do not think getting a free pass is healthy...you holding things in and making excuses for your husband is the very same things as walking on eggshells in this instance.
You are 100% correct but my head feels like a tangled mess. I'm trying to figure out what's 'holding back' and what's 'jumping on the triangle/blaming/making someone else responsible for my emotions.' For a while there I felt like I did nothing but talk to him about my frustrations. He wasn't able to take in the information so I moved to self-soothing and finding support elsewhere. I don't tell him everything I see anymore because it feels counterproductive. (This also may be my conflict-averse excuse - I'll chew on it.) I can't hold everything back, though, so maybe it's healthy to let some things through? Does that sound reasonable?
Article was great. I don't have any desire to take on the role of emotional caretaker for my MIL and the discussion helped me realize that part of the reason I instinctively don't want to is because she does not take any responsibility for her behavior. If there was a desire to improve or work on her part or do better, I would like to believe that I would put in equal effort. I can still use the info to help my husband be a healthier caretaker for his mom.
I am not saying he has BPD but he has some definite problems with differentiation and boundaries among others.
I've often wondered if he has BPD. Best I can tell, it's years of conditioning that may or may not be undone. It is still overwhelming, especially when I feel like I'm 'doing the work' alone. He comes to therapy, but he says it's to support me.

Thanks again, Harri. I always appreciate your thoughts.