The answer to both is yes. You'd still need a good gloss coat if you want the decals to go down without silvering. Any gloss left from the ink isn't adequate to cover the model well enough for decals. I'd apply it before washing, wait a while, do your wash, then wait a while again, finally gloss coat it once or twice more. That way your wash wont' go anywhere or be harmed by water and/or decal solution.
You should be able to spay gloss laquer through an external mix airbrush, and it will probably need thinning as well. Why don't you give Future a shot instead, it's great and tough and glossy, and it doesn't need thinning.
Madda
Trifles make perfection, but perfection is no trifle. -- Leonardo Da Vinci
Tact is for those who lack the wit for sarcasm.--maddafinga