I'm building this weapon and I was wondering if separate pieces merged as one object are acceptable in most game engines (and portfolios).

Yep that is perfectly fine to do.

Perfectly fine, but it may depend on the game engine in use. Some can handle it well, in others it may cause lighting issues, or count as a seperate draw call. I believe unity does or did count seperate pieces of geometry as an extra call. But it is pretty standard practice to model like.