1. I wouldnt expect any headliner to be structural, but if its keeping the bulkheads in place, I wouldnt mess with THEM.
2. Alternative to removing all the headliner, one could use a hole saw to access the underside of the cabin top and have much smaller panels to fabricate. and much nicer, if the fastening comes undone and the panel takes flight under way - people stick their families down there to keep them safe at times.
3. be careful with the insulation selection or you may get mold growing in it.
4. and whats wrong with shoving wires in between the headliner and deck?