I have never had a problem with urethane yellowing. I did a mailbox made from a sportster tank and it has been up for a few years and still looks good. The guy polishes it about once a month to keep it shiny. Just spray the first few coats dry. This builds a protective layer between the acrylics and clearcoat and eliminates any problems with compatibility.
For regular daily cleaning I use automotive laquer thinner. It's cheep(a couple bucks a gallon at Wal-Mart) and it works. Don't soak your airbrush body in it, that is what destroys the o-rings. I soak the tips and needles if they get a lot of buildup.