I know from experience - with Rusto - that you can indeed polish a flat to a gloss.
I tested a bit when I painted my Newport, and it was amazing how quickly the flat finish went shiny.
And I didn't use a single coat of gloss on the car anywhere, so it wasn't that I polished through.

I sprayed my flat finish, so I can't help you with the roller marks issue, but as above, I can say that you can make a flat into a gloss pretty quickly!