I walked out of my family home at 15. Sofa surfed, got a job, found a room. Went back to school.

Look I know at 16 your home is your entire universe, but best thing she can do is buckle down at school.
Do as many extra curricular classes as possible to save her going home.
In the UK she could report her dad for spending her social cheque and not providing her with the things she needs. But I don't know about the US.

Really there isn't anything you can do but focus on doing the best you can so that when the time comes and if you are still together you can help the best you can.

As for smart assed step moms just keep doing what your doing. Being absurdly polite and respectful.
I would take it further and ask for permission clearly before doing anything...even moving from room to room.
But be genuine about it....;-)

It's tough watching someone you care about hurting, but ultimately it's up to your gf how much **** she is going to take before she has had enough.

Just keep reminding her it's not forever, only 24 more months then she can get out.

Best of luck...hang in there.