I understand as my MIL's went a bit nutty while I was pregnant...both MIL and Step MIL both started competing over what time they would get with "their new baby" - at about 30 weeks, I lost my **** too lol
Sit down with DH and discuss what you both want...and then make DH call and speak with his mother and tell her to back off.
MIL has never babysat, and DD is now 2 1/2. Infact, she has only ever been left with a babysitter once and that was my mum.
See the porta cot as something for you to use when you go over there for lunch...don't feel any pressure to leave bub when you are not happy.
MIL is still a pita...when ever she comes over she does all she can to get DD away from me....when she was tiny she continually picked her up and walked out of the room....every.single.time...drove me nuts and I will be honest and say I stopped inviting her over. Even now, MIL tries to tell me about my DD...she sees her every few months for an hour or so but tries to tell me things as if I don't know her. I now just ignore, smile and nob and then vent to my girlfriend who understands.
As long as you and DH and united...just ignore what she says she is going to do!