Bit easier to search then. Unless the ceilings are finished.

This is a sound thing. Make the house as quite as possible and start searching the areas where the pipes run. You will most likely have to get in the attic to hear the leak. It should be fairly obvious. Especially if your finding blown insulation inside the catch can.

Its easier to replace the turlet. You would have to reglaze the toilet. Which I doubt anyone does.

Yeah, I'd skill saw it off. Set your depth a Little shallow of full depth so that you can pry away the cut piece.

Tbh, I thought it'd be much higher. Like closer to 90%

This should work. But you will need to scuff the epoxy as well once it cures. Use water or a hepa vac for this process.

Is the drywall physically dry? Obviously the less the humidity the better. But oil based paints dry differently from latex paints. Either way I'd dry the area as best as possible. And ventilate while using kilz. That stuff is rough in an enclosed area.

You will need to go to the city and see if its even allowed if you want to be proper. But id run electric first. And you should be able to do everything you wish. But idk if you can with whats there. As whats there looks like 2' on center and not 16" on center. Which can be a problem. But as it's not a dwelling it'd probably pass. Again ahj is the one who has say here. Ahj=authority having jurisdiction. Which is the city.

Proper ventilation is a must. And any heat source that doesn't have a open flame should be safe.