I don't have the altitude to deal with, but I do hike and camp in the Ohio River valley, where daytime conditions in the summer are temperatures in the 80s and 90s, and humidity in the 80s and 90s. In the winter, about the worst I'll go out in would be lows in the low 30s and humidity around 60 or 70 percent.

As someone else said, I think down's susceptibility to moisture is overrated. I've used down quilts in those conditions for years with no problems at all.

You might consider a system like this:

A down quilt rated to the mid-30s (I use a Thermarest Vesper 32, which has a comfort rating of 40 degrees.) As important as the quilt is the pad under it: I'd go with a r-4 rating or higher. (My choice: Thermarest XTherm regular, r-5+) I'd choose a quilt because they aren't as bulky as a sleeping bag (a synthetic quilt will be less bulky than a synthetic bag, too.) Also, they're easier to use: if you're camped lower and warmer, it's easy to reposition the quilt or stick your arms and legs out to adjust the warmth. I use my 40-degree quilt on 80 degree nights without any issues.

A down-filled balaclava, which becomes a hood to use with the quilt if you need one. Montbell has a really nice one in its Superior Down collection that I use.

Down-filled jacket (hooded or unhooded, your preference), down-filled pants, and down booties. You can wear these around camp before you go to bed, then wear them under your quilt to get another 10-15 degrees of warmth while you sleep. (How much depends on the loft of the garments. I've stayed toasty at 25 degrees in Smartwool long johns under Montbell Superior Down hooded jacket, pants and booties, using my 40-degree quilt.)

Another advantage to this "system" is that you can leave the jacket, pants, booties, and balaclava at home when you don't expect temperatures to drop close to or below the quilt's rating.

I can't swear this will give you the flexibility to meet the wider range of temperatures you encounter versus what I deal with, but it might nudge you in the right direction.