My thoughts on this:
-TG this is not my MIL. I am sorry you are having to deal with this.
-What you are calling a boundary seems to me to be more a punishment (not seeing us for a few weeks seems like a punishment.) A boundary would be, "If you give our son candy (or whatever it is), we will not be able to leave him with you."
-Her behavior is the expected blow-back from this. Part of it is lashing out in hurt and anger, and part of it may be legitimate criticism.
-It is good and mature of you to consider which of it is legitimate criticism, but it would be foolish to be completely transparent with her if you cannot trust her to not use this against you. What I mean is, behave towards her in a manner that won't make you ashamed, but also protect yourself. You owe her no explanations nor justifications.
-If you know what to give her that will stroke her, just try giving it to her without any expectation of anything in return. I know you have been resistant to doing this b/c of the obnoxiousness and hurts that have come from her, but sometimes you just gotta be the bigger person and see how it goes. Reevaluate later. Remember that she may never change, but she is going to be your MIL forever, so you gotta figure out some way to be that won't make YOU feel bad. She may always be a PITA, but at least you won't have to feel bad about your behavior towards her.
-Don't get drawn into discussions of feelings with her - she probably cannot handle such a conversation.