Yes, you can definitely varnish with Matt or satin, then go over areas with a gloss varnish (like ardcoat). Normally best to do this with brush to avoid over spray. Or alternatively you could gloss the whole model, then Matt down the areas you don’t want shiny.
100% yes. If I’m painting a freehand (like a marine shoulder pad), I will gloss the area first, then paint my design. If I make a mistake, and need to remove paint I can without fear of damaging the paint underneath. You can firmly rub off the layer with kitchen towel.
Some techniques are best with varnishing between steps, stuff like pin washing and oils etc.
I personally use an airbrush (as I find I get better control), I do a coat of Vallejo gloss, then a coat of testers dullcote. Depending on the mini I might re-gloss some areas by hand.