I'm not a step parent, so can only say how I would deal with my own children.
We were ok with our children having friends to say sometimes, but they had to ask and we would have hated it to become a regular thing. Selfishly I want to relax in my own home and that's hard to do if you have random strangers staying over. My daughter's boyfriend was the most regular visitor,especially when he was sofa surfing (he had a good reason for this) and he was always polite and considerate. I still didn't enjoy it, but it was bearable. We also requested that friends didn't stay when my husband was recovering from his regular cancer treatments.
The job situation is a total joke and I don't know how you stand it. I know this won't work with everyone, but we made our newly jobless child get out of bed, the rest of us went to church and she went around the city handing in a CV. By the time we met up at lunchtime she had had an informal interview and got a full time job. The deal was that if you want to live at home, you get a job and contribute. It is difficult now because 3 years without working and with no experience makes him very unappealing to employers, but it can't go on.
As for the music career, the boyfriend I mentioned earlier is a musician, so I can comment on that. He has been working at this for a few years since leaving uni, however he has always worked very hard at other jobs to earn money. He had to because he had no parents locally to live with and bail him out. Another difference is that he had a good chance of actually making it in a band and that has proved to be the case. His music career now pays the bills and they are doing incredibly well.
One more thing. I'm very kind to my children and find it easy to apologise when I'm in the wrong, but I wouldn't have put a sorry note under the door. It just gives him another opportunity to show his utter contempt for you. Smoothing things over is all very well, but it doesn't deal with the real issues.
I hope you can see your way through this and that your dss starts working on a better future for himself.