So our 18yo kid moved back in. Temporarily. Here's the deal...we for all intents and purposes got what we wanted: a apology, a making of amends, an ackowledgement that he was wrong and he'd fucked up, and a plan for how he was gonna change.
Rent (because he's not in school and we don't house freeloaders who are of age),
Clean his room and himself regularly (because no adult wears his underwear 4 days in a row and isn't living in a box),
Don't lie to us,
Don't steal from us,
Don't smoke or do drugs.
Really, not a difficult list, but one he was wholly unwilling to abide by. We'll see if he can handle it. So far, so good, but after living in squalor, anything is better the first few days.