The problems with damp insulation are very much overblown, they tend to be more fear based than real world. I was guilty of it for many years.
I've used both down and synth, for extended periods, on countless trips, in cold and wet conditions. Both are fine if your not being lax, or your kit has got utterly soaked in a flood/fall in a river etc.
I can't remember using any sort of cover for my sleeping insulation in over 2 years now, maybe 3...that's over 300 nights in the UK, lots in winter, in drippy single skin shelters. Sometimes, my down is a little damp in the morning if it's especially foggy, or it's been dripped on or sprayed all night. It quickly dries out the next night or with an airing in the day.
Down inside synth is fine, it works well as long as the weight of the outer layer doesn't compress the down. Personally, I wouldn't take an Apex quilt to manage the moisture in the down layer; it's just not needed. To boost the insulation? yes.
If a layer is needed to move the freeze point, your in serious cold and will likely know about not introducing moisture into the sleep system in the first place. If your getting condensation moving through the outer layer into your down, the DWR could do with a refresh.
BTW, If anyone has any of that nasty non-hydrophobic down kicking around in sponge-like sleeping bags and quilts, I'm happy to 'recycle' it for you.