Clear coat before decals?

Before so you have a smooth surface to apply the decals, which helps prevent silvering.

Both. I generally consider decal application to be part of the weathering process, which I always do over a clear coat than cannot be compromised by any weathering thinners or solvents, etc.

A final clear coat is a little more optional, but I generally do it to flatten everything out.

I generally use a clear gloss coat, to prep for decals, and then a matte clear coat to seal everything at the end.

If I am decaling over a gloss finish I do not bother to clearcoat before decals.  If the decals are going down on a matt finish, however, I always glosscoat.

I'm also in the before and after camp, and what GM said explains why quite succinctly;

GMorrison

A final clear coat is a little more optional, but I generally do it to flatten everything out.