Sorry you are having all this trouble. Getting started airbrushing can be frustrating.
I'm going out on a limb and guessing your problem might go back to your cleanup after shooting the Alclad II clear. I don't think Mineral Spirits is going to work, and I believe you need to use lacquer thinner. (because Alclad is Lacquer based......unless you used the acrylic Alclad in which case the mineral spirits would have probably created a horrible mess that you wouldn't be able to see).
Fouling our airbrushes with a clear coat or varnish is a bad deal becuase we can't see it. Acrylic, lacquer, whatever. I'd do a complete strip, and clean each part with lacquer thinner. Try again with the VA Nato Black Model Air.
Since you've shot Model Air before, maybe you have those basics down. A few observations; Model Air blacks are pretty thick, though you can probably shoot Model Air black neat through a .3mm nozzle, you're right at the limit IMO. You mentioned Liquitex. Some years back I experimented with using Liquitex as a retarder or flow improver with Vallejo and found it to have no effect. It perhaps made things worse, I never quite decided.
Just to make sure, you thinned the Model Air with what, if anything? And what air pressure?
At any rate, I think your airbrush should be getting the job done and I'd hold off on buying another right now were I you. Unless you want to, of course.
I'm no expert in any of this, But I have used all of the products you have mentioned (except your specific airbrush) and I wanted your question to be addressed. If any of my advice is bad, somebody please advise.