Huzefa and his crazy ideas. Actually I enjoy them as they do make me think about it in a different way. Let’s see if I can help any.
Mylar is heat sealable (I’ve done it). But, it needs a higher temperature. I don’t know if your iron can go that high, maybe it can. Space blankets are aluminized Mylar (meaning they have aluminum sprayed on one side). Normal Mylar is clear and also known as PET. If you heat seal a space blanket, you have to make sure that you seal the non-aluminum sides together, or it won’t work.
The poly bags that Bill used are LDPE (I think). They require a very low temperature for heat sealing, and have a low density (compared to many other plastics).
I don’t think silnylon will work. Your stuff sack test will show one way or the other, though. I think that the silicon that is impregnated into the woven nylon will be pushed out of the way to let the pressurized air out. This is pretty high pressure (a grown man putting his entire weight on the pad). The silnylon wasn’t designed for that much pressure, only for keeping water out with no pressure difference. I also don’t think that seam sealing will keep it air tight under pressure.
I don’t know about the cuben. I know you can heat seal it because it is a PE, but how air tight is it? Can it take the pressure?
Seam sealing your compartments is doable, but I would imagine a pain in the but, even with a custom heat sealer.
I spent some time looking over Bill’s design, and thinking about your original post on this topic. I always come back to his original idea being the best. If you make it one valve for the entire pad, then you can’t take out the down and dry it. If you get a hole in one baffle, it is easily replaced with a spare.
Another thought that you and Bill might not have considered. I haven’t looked at the calculations, so I could be wrong, but hear me out anyways. I think that you could use less down for the same warmth. In a sleeping bag, the amount of down is determined by how much loft you want, and how much that weight of down will loft (ie, hold up the top fabric to form a specified volume). In the case of an inflatable bed, the down no longer needs to hold up anything, just fill the space. As long as there are no convection currents, using less down should fill the same space as more. Anyways, just some theoretical wandering.