"Why am I self absorbed to want the birth to be something I get to decide?"
@Seline - I don't think you are - childbirth is one of the things in life we cannot control, usually - so it matters more that we do get to control the things about it that are within human control - if that makes sense. I did say earlier that I thought it was not unreasonable of your MIL to come to the hospital to support her son - but your more recent posts don't paint a picture of a MIL like mine - supportive and caring, and respectful of her DIL's boundaries - she sounds like a pretty difficult person, and given all that, I don't blame you at all for being cross with what happened.
I do hope that you can find a way of moving forward that allows you to put it behind you - I wonder if it would help to write a letter to her, saying everything you want to say, no filters - and then burn it. Lance the boil, as it were.
"As the mother of a son I'd be sure to keep a distance if that's what his wife wanted - pushing your way in is a sure fire way to end up one of those MILs who never get visited because you're such a massive PITA"
I agree with this, @GunpowderGelatine - I've got three dses, and as I said earlier, I can easily imagine wanting to go and support them at the hospital, if their wife was being rushed to theatre (especially at only 26 weeks pregnant when the risks to the babies and the mum are clearly greater), but that support can and should be given without overstepping the daughter in law's boundaries. Wait outside theatre with your son, be in the waiting room - but what the OP's MIL did clearly went way over the line.
My PIL came to the hospital to visit me and ds1, within hours of me giving birth, and later on, they took dh home and made sure he had a meal (this was in the days when visiting times were limited, and I had had a very long labour so wanted and needed to be left in peace - they weren't taking him away to make sure he was fed and rested when I still needed him).
But the key difference is that my PIL knew my boundaries and respected them. My own parents couldn't be there - they lived too far away - and anyhow, my relationship with my mum was never that close, and I had a great relationship with my MIL.