My First Name I totally agree. OP - your baby, your rules about what happens to him.
MIL's house, and she can do or plan what she likes but your baby, your rules.
I also want to say, when my first DC was this little I was constantly made to feel overprotective, irrational and stupid when I was anxious about her being passed around, or anxious about her being out of routine, or just anxious.
For instance, I let SIL (she's very nice but has no kids and no experience) tell me in a condescending voice that my 3mo 'could cry for a bit longer before I picked her up'.
I am so angry about this now. I believe we all have different levels of emotional response when we have a baby. Some people seem relaxed about hacing their tiny babies handed around; some don't. Both are fine. I believe that it's right and proper to feel very anxious and concerned at times, especially in a different environment like MIL's house.
It's that anxiety / concern that drives us to be good, careful parents - it's what stopped our ancestors' babies from being eaten by tigers!
You don't need to justify anything to anyone here. If you feel upset by your MIL's behavior, you feel upset. You have a newborn and your family should be especially gentle and careful and caring with you, and especially respectful of your wishes. Sadly, that probably won't happen...
So stick to your guns with your little boy. He's YOUR baby, you're his mum, and YOU KNOW what's right for him.
Good luck - and FWIW I have a bit of a tricky relationship with MIL and the DC have made it so much easier, as now we both have something to focus on - so in future it might all be for the best.
Sorry for all the bold but as you can see I feel strongly about this!