So while I'm fully against the unannounced drop ins, people commenting seem to be upset towards you in an odd way. I do think you shouldn't go over to her house anymore, next time text your BF to meet you somewhere close by, and don't go into her house, it sucks you're not welcome where your BF lives, but it's her home and her choice ultimately. If you end up partnering with her son, she wouldn't be welcome to drop by unannounced to hang out with her son, you wouldn't accept hiding from her in your room as an option.

Why and why did MIL block you?

What is your BF's plan to get out of her house if she's not allowing him to have his GF over?