X demands full custody because I'm now engaged

[sleeper]

I was posting over on Figgeroni's thread, "X's are like a virus..." but I see I need one of my own.

Recent events:

1) I propose to significant other of several years in July.
2) X calls next day informing me I should have talked to her first (wtf?!?) and she had already talked to a lawyer and would be taking "full custody" of our children. There's no logical reason for such other than X is jealous of fiance's relationship w/children and/or me (X asked to see pic of engagement ring when I next saw her and I have concluded she's basically crazy).
3) I return to town, X makes nice for awhile.
4) School starts Monday, she expects kids to begin living with her (their grades/education is her faux-excuse).
5) I said "no"
6) It's "ON"
7) We have "joint, shared custody." In our state this is perfectly 50/50 split. I am the legal domiciliary parent of our son, she is the DP of our daughter (I think that has to be stated for tax purposes but could mean more).
8) We have always shared the kids week by week (one hers, the next mine) and until now have been the model for co-parenting post divorce.
9) It's ugly now (she isolated DS from OMH and DD and drove him around today interrogating him because he asked me what I would do if I came to get him and mom wouldn't let him go and she had a gun).

I'm sick because she put my son through that. She ended up taking him to a "family counselor" she located on the fly. That's her style because she believes she doesn't have any problems, so such a statement must indicate that other person does. When I stopped answering her calls, she had DS call me repeatedly and was telling him what to say to me (I could hear her in the background).

My son is only 11.

OMH generally seems like a decent guy but she's got him snowed or highly manipulated.