Hi K.! Congrats on your new baby boy!!!
I think what your describing is pretty normal for new Mom's. I have a 2.5 year old and a 4 month old and they both stayed at my MIL's house for the first time this past Friday (We had our basement water sealed and it smelled too bad to keep them in the house overnight). I was definitely nervous but now that I have a couple years of parenthood under my belt I was able to relax a little and really ended up enjoying some alone time with my husband. And you know what, we picked them up Saturday, they were well cared for, got spoiled and had a GREAT time with Grandma and Grandpa.
Your concern probably stems more from control then it does from you MIL. My mother in law is a lovely woman who loves my kids like nobody's business but it took me 2 years to allow them to stay overnight. Let's face it, our kids are the most important things in our lives and she's not YOUR Mom and she's not YOU so it's natural that you'd be reluctant to hand over your most precious treasure! But if you step back a moment you'll probably see that she gave you your husband and hey, he's a good enough guy that you married him and had a baby with him and he survived into adulthood so she hasn't done that bad as a caregiver right?
And who says you have to leave your son with her. If you don't want to then don't but if you would like a night out and feel good in knowing that someone who LOVES your child is watching him then I think you should give it a go. Also, who says it has to be at her house. We rarely take the kids to them, we ask them to come to our house. It just makes sense, the house is already babyproofed and the kids have their toys and beds etc. Maybe test the waters a little. Have her come over, go meet a friend for coffee or go shopping for an hour and eventually you'll feel better. If there are safety concerns, state them to you MIL in no uncertain terms. My MIL kept talking about how we all slept on our tummies and what's the big deal. I told her flat out, as kindly as possible but very firm that if she ever put my kids to sleep on their tummies, she'd never be allowed to be alone with them again. That's all it took. If it's not going to affect their health and safety then let it go. She's going to do it her way and every once in a while that's ok. I told my MIL not to feed my son in the middle of the night......guess what, she did but she did so because he woke up, wouldn't go down and she was tired....can I really blame her? Grandparents are supossed to be fun, let her have that and let you and husband have some time together without the cost of a babysitter and the uncertainty of someone that isn't related to you.
Good luck and listen, it took me over 2 years for an overnight visit. Take it as your comfortable, they are your kids and you know whats best for them.