I was ok in a 20 degree bag in the Whitney area in July last year but it froze at night and we had 2 days of rain and some hail so was generally cooler.  Oddly I found that wearing a long wool base layer moderated the temp and I was much more comfortable than wearing just a tee and briefs.

The rest of my party continued on to Yosemite on the JMT and it was clear and much hotter.  They had quilts...an Enlightened Revelation 20 custom and a Katabatic Flex 10 (15 overstuffed I think) both of which suited them for the whole trip.  We all used X-lite inflatable pads although I think they also took Z-lites for extra sleepage.  I'm considering a Feathered Friends Flicker 20.

Sorry to hear about your Duplex.  Disappointing for such an expensive piece of gear.  I guess they are optimized for 6mth long trail thru hikes which will wear any tent to a nub and not for leaving to your grand children. 

I had fairly good luck with the longevity of my now ~8 year old gen 1 BA CopperSpur UL2.  Admittedly it has only seen occasional use in that time but I have lent it out a few times.  I did notice the seam taping was starting to de-laminate in the fly when I washed it after lending it out last year but it wasn't leaking.  I patched the worst areas with silicone seam sealant.   So far I'm quite happy with the Tiger Wall UL2 that I now use although it did require some minor modification to the fixed guys.  I hope it lasts as long.