Headliner Shell Fabrication

Yes, you are correct fiberglass resin will melt Styrofoam.
Urethane foam will hold up and cuts and sands just as easy.
I glued the urethane foam blocks together with liquid nails.
Aluminum furnace tape can be used also where needed.
I did a model a ford headliner in the method your planning many years ago. I don't remember it costing much as far as the materials but it took a good deal of time.