Those are some tough calls with eldest O. I guess I'd say give her the chance with school, that's totally reasonable as both her mom and the trustee.. Having a housing crisis in the middle of a semester is not conducive to staying on track academically.
But truthfully I'd also say she probably knew if push came to shove she'd be able to extract more money from the trust, and this may have contributed to the lack of urgency with which she's searched for a job.
I think making it clear (which it sounds like you're already doing) that the safety she has will only be there to help if she's actively helping herself is the right way to go.