I personally wouldn't even bother replying to mil.
Her actions may have been well meaning and she got it totally wrong. She could have asked your bf if he liked the bedding and does he want her to change the bedding to surprise you and he could have said yes and encouraged her.
She could just be a bitch.
It doesn't matter why she did it really because your bf is the one who is mostly in the wrong and it's his actions you need to concentrate on. If he was to respect your feelings and put boundaries in place then she wouldn't be able to do this stuff. He would have told her and if she was being kind she would realise that you don't like people going into your room and changing your stuff, and if she was just being a bitch she would know that together you would not allow it.
She can only act this way because her son allows it.
You can stand up to her but if she knows she has her son's backing she will just continue to push the boat until it interferes with your relationship. You will end up the bad guy. You can't 'win' if he isn't on your side.
My mil was very interfering and it caused huge problems between me and my husband. I could have stood up to her all I wanted but she would continue to do her shit while her son enabled it. It wasn't until he said enough was enough that it stopped.
Your bf has shut you down, told you it is his problem, and accused you of being irate. This does not bode well. If he doesn't have your back then this relationship will not run smoothly. I would worry more and concentrate more on your relationship than I would mil. When I started focusing on what my husband wasn't doing rather than what my mil was doing things got better. It was easier in a way to focus on her rather than the fact that my husband was letting me down and allowing me to be treated poorly because he didn't dare rock the boat with his mother.
Thankfully it is all sorted now, but not without a lot of tears and resentment on the way and eventually completely ruining his relationship with his mum.