I think you nailed it right there. Bugger, should have thought about that. Exactly that, gloss underneath, the decal, followed by lashings of decal fix and then an overall matt varnish for a finish. New approach, sod the gloss and go for a matt varnish top and bottom and use the decal fix to get good adhesion.Apologies if this is incredibly obvious, or teaching you to suck eggs... and I only say this because it looks like it's the underside of the decal that is reflecting...
...but did you put a gloss varnish under where the decal was going, so that it's a perfect contact underneath, then apply the decal, and then put the matt varnish on top?
I now feel like stripping some of them and starting again. Not really in the mood for that as it will take a fair bit of work, don't see many alternatives. I can try smearing them with a film or something or other but it won't change the physics of the layer underneath. Maybe try some experiments first, it can't get any worse.