Putting decals over dull coat results in the decals "silvering" from air trapped underneath the decal film by the matte finish of the paint, which itself is created by small "hills-n-valleys" in the surface of the paint, which scatter reflected light and makes the surface appear "dull", rather than "glossy" paints which flow out into a "near perfect" smooth glasslike surface. Decals applied over glossy surfaces will not (SHOULD not) have any trapped air underneath them (and yeah, while you put on waterslides with water, therefore there SHOULDN'T be trapped air even on matte finishes, when the water evaporates away, air takes it's place under the decal on matte finishes, leaving them looking "silvery" and standing out (fugly) on the surface of the model's paint). Decals put on glossy paint snug down firmly to the glass-like surface and "disappear" into the surface.
Once the decals are all on and look good, then a coat of "matte finish" clearcoat goes on over the glossy paint AND decals, both protecting the decals and underlying paint AND giving the "hills-n-valleys" finish that scatters light and makes the "dull" look of a matte finish...
Get it?? Not a matter of doing things "the hard way", but rather "the RIGHT way"...
Later and good luck! OL JR