Not sure if I can "enlighten" you any more than you already are, but I struggle to see why anyone would choose synthetic...however I have never had to sleep in a puddle in 110% humidity for days on end, so your miles may vary ;)
To me that 110% humidity is the important deciding factor. At least once per year I endeavor to wash my down bag, and I assure you it is not an easy task to wet out a down bag, even when submerged in a bathtub. In continued subzero temps, the use of a VBL is good practice no matter what insulation you choose, so again moisture in down should not be a cold expedition issue. Down is lighter, more resilient, and just plain feels nicer the way it drapes around your body.
I don't tarp, and I always carry a bivy bag, so using down is in no way risky in my hands. But if you tarp in humid conditions without a bivy bag, you may find synthetics a better bet. Or if you really HAVE to sleep in puddles.
I have been puzzling over the WT3 requirement for synthetics too. I still don't really understand the rationalle, but I have given up trying. The organizers decision is final, and no correspondence shall be entered in to. I would have thought weight would be the deciding factor, and towards THAT end I would have thought that a couple of 4-person down quilts would have trumped any other option for weight:warmth, but I'm a chick and we're not quite as adverse to sharing body heat with strangers as some guys can be...