Whilst also trying to diagnose my DP2 FFF printer, I finally tried something I thought could work better than having an air gap for the front decal. I exported a second STL and was easily able to merge the bodies and change the relevant settings to get the effect I wanted:

It's printing now and perhaps I should've gone with dual thick infill so that the lines didn't cross over each other. It all seems to be sticking fine (and going much faster than dual lines since the printer decelerates much less often) so I'll leave it.