I feel your pain.
Your MIL sounds like my mom. I don't think my mom would actually try to barge in anywhere, but she definitely seems to believe grandmothers and mothers are equal and has little to no regard for my feelings/wishes around birth.
BUT -- this is our fourth and our last, and things are very different from when our first was born. Fortunately but unfortunately, my mom and I are not currently speaking (mostly imposed by me), and on the rare occasions we've communicated, I've made it clear I don't want her around for the birth. I've no doubt she is terribly offended by this, but y'know, for once I am truly protecting myself and putting myself and my family first, so that cookie will have to crumble where it may.
At the same time, having a fourth child is obviously going to necessitate some help for us. Especially because I must have a planned cesarean, so my recovery will be a bit tougher. We have asked DH's mom to come for two weeks, and that may even turn into more, depending on a variety of factors. We're also temporarily hiring a sitter, reaching out to friends and community and basically trying to surround ourselves with as much help as possible in the weeks after the birth.
That said, last time my MIL was here she took pictures at the hospital, promised me they were just for me, then promptly sent them to her brothers. So this time, DH is going to tell her to leave her camera at home but that we would love her to take pictures with *our* camera, if she would. She actually has a pretty good eye sometimes, but like you I feel it's a vulnerable time and I want to be able to control which pictures go out and to whom.
As for hospital visitors, she will probably bring our other kids to visit the day of the delivery -- they all are anxious to meet the baby and I will be tired but I can manage that, especially as I said provided she doesn't have her own camera ... but, there's really no one else to visit us besides a couple of DH's siblings, maybe his dad and dad's girlfriend ... and, I could ask DH to tell them all to stay away but frankly I find the hospital stay a bit long and lonely and as long it's not someone emotionally upsetting to me, I enjoy (brief) company. My mom would be upsetting, though, and it sounds like your MIL falls into that category.
Honestly, bottom line, your DH needs to say very simply that you two don't want visitors in the hospital and that he will let her know when you all are ready for that as a family, after you get home. If you feel at all concerned that your DH won't be firm enough, or that your MIL will disregard whatever he says anyway, definitely speak to the staff as soon as you check in and make it clear you want NO visitors AT ALL except your husband -- AT ANY TIME. Seriously, they cannot just let people walk into your room without permission, that's just crazy.
GL and let us know how it goes!