1. Dry transfer is a pain in the ass, the bigger they are the harder to get it right. Its the easiest among the 4 types of decals/stickers to be ruined while applying. Look up tutorial in youtube to c how u can apply this.

2. Depends. Top coat has 3 kinds of finishes, gloss, semi gloss, and flat. The optimum surface to panel lining and decaling is gloss, if u plan to apply it straight on to bare plastic u dun nid to coat anything beforehand cuz gunpla bare plastic is gloss enough to begin with. If u prime, paint or sand ur kit then u should gloss coat it 1st before panel lining and decaling. At any rate, u must coat it after panel lining and decaling with whatever finishing coat u wan.

bare plastic --> panel line / decals --> whatever finishing coat u wan
bare plastic --> sand / prime / paint --> gloss coat --> panel line / decals --> whatever finishing coat u wan

Of cuz the above is the optimum way of doing things, its kinda time consuming and cost alot to do so. I kinda take shortcut whenever i can, up to u to experiment on where u wan to take shortcuts.