I think your 50/50 is pretty awesome, btw - it stinks that my H is barely bonding with our son. I would hate it from a selfish standpoint (how would I feed him every other night, etc), but I think it's probably really good for your son. AND you shouldn't be penalized time with your son for what she did.
I was more just saying how I understand what the default views on it might be and how she might try to use it as leverage. With her crazy scheme w/ OW and your clear love of son, I would avoid giving much ground here at all.
Faith is, at one and the same time, absolutely necessary and altogether impossible. --Stanislaw Lem