Best finish advice needed.

A single stage paint doesn't require a clear coat to obtain the gloss. It has nothing to do with if a hardener is used or not. It's a term that has come about since the use of basecoat/clearcoat systems where the basecoat provides the color but is relatively flat and requires a separate clear coat to get the gloss. That is sometimes called a two stage paint for that reason. All urethanes use an isocyanate-containing hardener. That includes the single stage paints and the clear coats used with bc/cc systems. At one time DuPont marketed a non-iso clear. They may still have it. I have never used it so I can't comment. The vast majority of clears require an isocyanate-containing hardener

Be aware that the hardeners used in two-part acrylic and synthetic enamels also contain isocyanates. You can avoid isocyanates by using the tractor paints and some automotive enamels that do not require a hardener additive. Of course you loose the advantages of using a hardener. You can still get a very good gloss but fixing errors are a problem and the chemical and fade resistance and gloss retention will not be as good. You are correct about avoiding isocyanates unless you have a fresh air breathing system. Getting such a breathing system should really be the first priority for anyone serious about using modern paints.

Rod