Greg
The cabin liner will remain a resident, though, like yours I suspect, it doesn't fit that snug to the cabin top. That loose fit was the cause of much distress to me early on before I discovered the soft, flakey plywood at the base of the main bulkhead. Now it just lends to the character of 113 This liner currently has alot of holes in it and in the end will have a alot of new ones in it from hardware and such. It would be nice to have a clean, continuous headliner in the main cabin not accented by acorn nuts. As to what's under there, I cut some large oval shaped holes in the cavities behind the port and starboard nav lights on the side of the cabin top so I could lay in some fabric and matting to close up the holes left by said light removal. That was the best look I've had at the underside of the cabin top. It appears to be of the same texture and quality as the v-berth area. Keep in mind that I've only seen an area about 40 square inches. I think removing the cabin liner could be a bit of a bear when you get to the underside of the deck where they attach to each other. I'd probably cut it out in sections taking the areas where it floats freely first and then work back to the points of attachment. Those 24 grit sanding(butchering) discs are efficient at fiberglass removal.
Just as an aside-make first couple of pieces of equipment you buy a good respirator and goggles before so much as sanding off some paint.