repeated capsizing

Not sure about the Capri... but most of these smaller boats have the deck sealed to the hull in a permanent manner. Some Sunfish have had the deck successfully separated from the hull and been resealed, but its not something that I would recommend trying. My bet is a professional shop would just tell you that it can't be done.

Typical for gaining access to do the modifications you list would be to cut holes and install "inspection ports"
Its pretty easy to get up to 5 inch hole saws and a 5 inch dia (nominal... actual hole you cut may be 5.5 inches) port is under $10. Cut the hole and do the work then install the port since the ring will reduce the hole available to put your hand through. Covers either snap in or 1/4 turn and then lock in place.
Lots of threads are in the Sunfish and Laser sections about installing the ports and doing work through the holes in the deck.

Ports are also available in other sizes including large enough to crawl through.
Duly noted about the transom. Now could I get some kind of pressure injected foam that could be shot in through a small access hole in a fairly large quantity while the boat was level? If the foam was stiff maybe enough could be injected providing sufficient body to hold level in the needed area and just let the rest go where it goes. I also wish it had the larger trailer line eye on the bow like the newer versions. Mine is part of the plate that secures the forestay, a poor angle for the line to draw the boat on to the trailer.
@aquaman did you ever do this? Are you happy with it? Does it seem solid? Any pictures of the process?