Simply inform them that you'll spend the night at your bf's. That's part of what it means to be considerate.

Maybe it's unfathomable to you, but in all likelihood they actually care, and were worried about you so they raised hell. Why else would they phone up everybody and their dog. They wouldn't take the effort of raising you and pay for your bloody living costs if they didn't. The same applies to your mother's.

Your mom is a grownup and responsible for her own decisions, including for taking decisions that led to her still needing parental support. It's her choice to never have grown up, to still depend on her parents. The only one to blame for her inaction is her.

The only ones who look good in this story are your grandparents. You're still half a child so you get half of a pass, but your mother does not.