It doesn't sound like they trust mom with much of anything, let alone parenting, given her addiction/mental health issues, and it doesn't seem like OP trusts them enough to let them know boyfriend's phone number / identity.

it sounds like they did call her brother at an hour when her brother would pick up, and he let them know she was at her boyfriends. in the absence of other information, they probably still expected that she would come home, which is why they didnt do anything else until 4am.

but yeah, hopefully it works out. i think OP is confusing "grandparents disapprove of my decision to stay at my boyfriends" with "grandparents think i might be face down in a drain pool somewhere."