The laquer thinner was a good idea and should have done the trick. If it were me I think what I'd try next would be to hose the spray regulator down with penetrating oil (something like WD-40). It will certainly need a thorough cleaning after that though. Then, instead of the little wrench supplied I'd try a "Real" wrench that would give some leverage. Like I said though, that's just how I'd do it. There's a chance you'll mess something up though, so be careful.
Have you ever had the spray regulator off before? If not there's a possibility it got cross-threaded from the factory or something.
The needle bearing is way back in the body of the brush and you'd normally never see it. I didn't think my Omni 4000 had one until I looked doen the back of the body with a strong light and saw it. It is aparently sintered in place and the reason it has a lifetime warranty is that it isn't replaceable by us.
Edit ... I looked at mine on the way out this morning, and a 9/32" wrench seems to fit pretty well although it's slightly loose. If you don't have one I'd take my airbrush to Sears (they have individual wrenches for sale) under the pretense of finding one that fits the head. When you find it just give it a good twist and then say, "Oh, well, guess I don't need it now!"