For the most part, correct on both counts. However, in general adding items like fleece jackets or even down vests, parkas or sleeping bags or quilts have been used to generate huge increases in the insulation. But there will be a tendency for the HH UC to compress whatever insulation is in the UC, in some places worse than others. Having something in there - like a pad - just heavy enough to slightly overcome the elastic tension creating a gap that down lofts up to fill can work good. I have had great results with having both a fleece jacket and down vest in the UC at the same time. The condensation caused by the Sil-nylon UC is overcome by using a space blanket and/or VB clothing or liners on the up(top)side of the down or Climashield.
If the UQ is between the hammock and the UC, I wonder if the UC would overly compress the down UQ to minimize effectiveness - and I'd worry about condensation inside the UC.