Nothing to be sorry about. I've got a signed/notarized MSA, so if nothing else, that's a binding contract stating that we agree that I have full custody. It seems tricky to find specific info on topics like this, but from what I CAN tell, she signed it, and it can't be undone. D or no.
Please please please talk to a lawyer. 99.9% will do a free consultation. Use that. You need to know what your options are. They've seen it all.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018