First, I qualify as an amateur finisher - I have done 20 instruments in lacquer and am starting to feel pretty good about what I can accomplish at home. I've also done one in a gun stock oil called TruOil. Lacquer can be buffed to a very high gloss where the TruOil gives more of an organic semi gloss finish.
Second, there are dozens of different finishes used on guitars - part of the problem is figuring out what can be done at home with materials available. For example, many manufactured guitars today use catalyzed finishes that are simply beyond the home builder. For most of us at home, to get a really high gloss our options are either solvent based lacquer (nitrocellulose) which can be applied from rattle cans, or so called water born cross linking lacquer (KTM, Target, several others) which can be sprayed or brushed. Oil finishes are fine but they will appear more like furniture than a PRS.
Third, I have experimented with several different pore fillers and depending on the type of wood I feel that some are better than others. I took a piece of highly figured koa and masked off three sections, one I filled with paste filler (a StewMac product), one with CA glue (use breathing protection) and one with an epoxy (Zpoxy from LMII). My feeling is that for "normal" wood like mahogany or rosewood the paste filler works just fine, for highly figured wood the Zpoxy really does pop the grain. Here is a koa guitar with Zpoxy and water born lacquer (KTM-9) over the top
Next you mention trying to highlight the grain with some sort of black stain. Not knowing anything about your wood I can't say what might work but it is pretty common with figured wood like flamed maple to apply a stain that soaks into on part of the wood more than another. This mandolin had a dark brown stain applied to the wood and then sanded back leaving it in the lines of the flame
Warning, applying stain is pretty irreversable.
What ever you do, make sure your materials are all compatible. Always practice on scrap.