I'm not having the borders issue in UE4 so it's probably something in marmoset. Marmoset may not be as lenient as UE4 for values between metal and non-metal. I believe in UE4 values in the middle interpolate the results of the two models.
The only practical difference between gloss maps in TB2 and roughness maps in UE4 is that UE4 uses an inverted scale, where black = glossy, while TB2 black = rough. In TB2, simply click the invert checkbox in the gloss section and you have a "roughness" map instead. There may be some minor differences in the exact mapping…
UE4 certainly has energy conservation, its built in to the roughness compontent. Tighter highlights are more concentrated and intense, while darker highlights are more spread out and dim (the same amount of light is hitting the surface, its just a question of how its scattered). The metalness workflow essentially has…