This is your child and you are resopnsible for his safety...which you obviously take very seriously, as should be. If you do not feel comfortable leaving him with MIL (or anyone) then you need to follow your instincts. Yes, children do not have to be wrapped in bubble wrap for protection, but as you know even simple accidents can have big owies. And vigilance does not diminish with the second, third or fifth (according to DFr who has 5, I only have 2).
As for your guilt of DS' accident...it was just that, an accident. DH didn't intentionally fall asleep before DS nor were you careless in doing your studies...you and DH are both responsible for DS and should care for him equally (imo). Accidents do happen and some are worse than others...we need to learn for them (which it seems like you and DH have) and move on.
MIL seems as though she would not have been happy with any woman your DH chose to spend his life with. Holding on to the past is just going to give you the stress, cause it's pretty obvious that she doesn't care much if she offendsyou or if you like her. My advice would be to just be as civil as possible with her. If she snaps and you feel it best to snap back, so be it (that is how I am

), if you want to let it run off your back then approach it that way. She may be jealous that you are now DH's everyday focus...one less to bow to her...so she takes the "opportunities" to they to pull him back into her disfunctional web.
It sounds like you and DH are on the right track now...on the same team. You need to "fight" the same battle, not each other. Ultimately YOUR family (you, DH, & DS) is your (you & DH) top priority.
And thanks for the update that couple who take wedding pictures before the cermony are doomed to divorce. Can you tell me when this is as I would really like to warn DH.

...nearly 7 year and going strong. Amazing what strong committment and communication can do.
Good luck on whatever you decide. Maybe limit visits for now until DS is a little older...not that he then wouldn't need supervision, but at least he wouldn't be as dependent for his needs to be met.
Enjoy your time as a SAHM. Preschool/PK isn't mandatory so if you (& DH) can/want to keep DS home you should, no matter what anyone says. I put DS in school at 3 and now regret not having the time with him. DD went to nursery school at 2 last year. This year I kept her home...I was a teacher so she is still learning, just cheaper and a whole lot more fun.