"But my question is: "Why would a cluster of bees, with the queen in the middle, be attached to the underside of the lid this time of year???"
jen, perry is on the right track here.........
1. bees will move up towards the heat and/or feed if they have run out below and are not moving side to side.
queens can and will be found on the underside of the inner cover when weather warms and the bees have moved up.
2. more importantly, this cluster appears to be very, very small, and not enough bees in this nuc to survive.
i don't think moisture was the issue, i think it was the # of bees, gauging from your picture jen. some moisture will be found on bees, after they expire. if they are soaking wet and frozen, and/or ice buildup on the side, yes, moisture. you probably wouldn't get ice build up jen. but take a look at the moisture on the bottom of the nuc, how much is there? are the bees 'soaked'? from your pix the cluster of bees don't appear to be soaked in moisture, just those near the sugar brick.
i also don't think you had an extensive 'poop problem'. if this nuc had nosema, one would see the tops of the frames and the face of the frames with dysentery all over them and it would not smell very good.
my guess is, this nuc didn't have the bee population to survive jen.