There's no need to strip the boat. There's going to be areas where you just need to sand the varnish with 220 and re-varnish. And there will be areas where you want to use a more aggressive paper to remove it. Perhaps 150.

I'm a big fan of wet and dry paper where you just need to just re-varnish. It just seems to cut like a fresh piece of paper forever when used wet.

Varnish is not hard like epoxy and you will cut into it or through it quite easily.

I would avoid chemicals. You just never know how the next varnish will react on it no matter what you do.

The big question always is: how do you know when all the varnish is gone if you have to re-epoxy a section.
You don't. I have done plenty of repairs. I have sanded down to the epoxy and feathered off on to the varnish. Did my epoxy repair never really knowing if my epoxy ran on to the varnish feathered off area. I never had a problem. I am totally convinced that epoxy sticks to sanded varnish just fine.