:So will the epoxy "peel off" with application
:of heat,

More or less. You have to imagine peeling a sticky lable off a plastic bottle.

:or does it just turn mushy and then you have to scrape it off in
: a big gooey mess?

You regulate the softness with the heat. When it cools it will be hard again(although ruined as a structural material I believe).Be sparing with the heat.I really like the Freud electronic controled heat gun. It runs from cool to burn in a smooth adjustment. Use a putty knife type paint scraper with the end of the blade sharpened square to get an edge of the glass up and then pull by gloved hand.

:This is not going to be fun, I can see it already.

Actually it is kind of fun. Or at least I find it more fun than sanding.

Once the glass is off use hook type scrapers on the remaining epoxy with and without heat. Keep sharp with a few strokes of a fine file every five minutes or so. Flexible cabinet scrapers are good when you are at the more rounded bits of the boat.

It's all good once you are back in the boat!