Re: painting non gloss paint
I wouldn't use flattener unless the surface is rougher than hell and you don't want to fair it, or your trying to do a match or blend job. It's much easier to keep clean and is a lot more durable without flattener. That flattener can turn the surface almost chalky if you use too much. The nonskid will break up the shine on the decks.
Re: painting non gloss paint
The cockpit bulkhead and door are painted gloss. All the vertical surfaces on the boat are. As he mentioned, if you use nonskid for the decks, that will break up the shine enough. I would not use flattener unless you absolutely need it. It makes the paint age faster and hold dirt.
Re: painting non gloss paint
It is all Gloss except maybe a little in the decks as you have done.
Good Luck