Compressing IX defeats its insulative quality. Since your IX is already in a semi-uq form ("4 layers sandwiched between ripstop"), why not just use that as a uq? Add a suspension system of shockcord and hang it under the hammock.
Essentially, a underquilt is a second layer. But its construction/suspension system avoids the structural issues you speak of.
Whats your weight? That will have a lot to do with what fabric will perform best.
You'll need to dart the individual layers of IX to get the optimum performance, as well as the top and bottom layer of fabric that covers the IX. Each layer being a smidge bigger than the previous. There are several threads dealing with that.
But you need to allow the layers to gap or open, once you slide it into the
2nd layer you kinda defeat the darting.
Trying to keep the IX quilt in place in between the two layer hammock will be problematic also. It needs a suspension system or something to hold it in place (velcro, ties, snaps?) Without a way to hold the quilt in place, everytime you move, the quilt can/will move and bunch up. Pads are firm enough to somewhat decrease this movement effect in the pocket.
My opinion, build a breathable hammock and suspend your quilt below on its individual suspension.
As far as condensation goes..theres probably a reason most of the major manufacturers use breathable materials as hammock fabric. Use your judgement on that.