Those silica gel packets are clever, but I’d definitely be cautious about them breaking open—what a mess that’d be, especially in the dark! I’ve used these bivvy sacks for garden overnights and the condensation does build up if you’re zipped in tight, but I just crack it open a bit at the top. Not ideal if it’s pouring, though. Haven’t had mine tear, even after some scrambling around on rough ground—mylar’s tougher than it looks. The noise isn’t my favorite (bit like rolling in a bag of potato chips), but honestly, you get used to it. 
As for padding, I usually slip an old foam kneeler underneath, mostly for my back, and haven’t noticed it affecting warmth. Actually felt toastier, maybe because of the extra barrier from the cold ground? Anyone ever tried stacking two of these or layering with a wool blanket for real winter nights? Curious if that’d be overkill or