bear,
It is so hard for me to say what I would think or feel, because even if we all have relationships with N mothers, yours is unique. I am tempted to say, "Take the beach house and when she tries to hold it over your head, remind her how much she begged you to take it and how it was a GIFT!" But honestly, I don't know how well you can avoid feeling guilty (which would be getting sucked back into an unhealthy interaction with her) when (not IF) she holds this gift over your head.
My husband is a healthy person but honestly he will tell you that he has a bit of N himself. (Just the fact that he has always recognized this in himself and consciously works on it tells me that he is not a real N). Anyway, he has the ability to let his N mother's comments (designed to make him feel guilty) roll off of him like water off a duck's back. While I will get mad at my Nmil's antics, he just says, "Oh, that's just mother" and honestly ignores her! So I can see how for SOME people, they could take the gift your mom is offering and totally repel her efforts to suck you back into her N world. If you can do that, I say take the darned beach house. if you can't ... NOTHING NOTHING NOTHING is worth your peace and stability.