I am not questioning Op's story.
I am questioning MIL's terminal diagnosis and the sources where OP got it from because sadly, some people use medical conditions to force their agenda.
Cancer/heart attacks/other serious conditions have been faked, exaggerated or just insinuated to exist to pull back the control they lost.
Yes, he is a mommy's boy as he puts MIL first. He makes decisions with his mother, not his wife.
Major decisions as child care, how to spend family time, where to live, are taken by dh and his mother and OP is supposed to happily obey and have no input at all.
Dh and mil are treating OP as the other woman and incubator
I bet OP would happily allow her baby to have more time with mil if they had a better and more respectful relationship.