Just replaced a GFCI and had two bad ones in a row, different brands from different stores. Third time lucky... :)

Guess what used to be a common additive to drywall mud...

I'm pretty sure OP does NOT want to peel off that tape without being ready to completely replace that pipe.

The stuff on the floor looks like it may be some sort of epoxy/sealer to keep the leaky pipe from running down.

https://www.google.com/search?client=firefox-b-d&q=nuke+it+from+orbit+only+way+to+be+sure#fpstate=ive&vld=cid:9de58e27,vid:aCbfMkh940Q

When I think sunroom, I imagine lots of windows. Triple paned, argon filled, coated windows are still only ~R5. Older double-paned windows, are probably closer to R2-R3. (Typical 2x4 fiberglass insulation is like R12-15). Closed cell spray foam is ~R7/in, You don't typically fill the wall space 100%, so Maybe R18.

If the windows make up, say 50% of the wall, your whole wall will only be ~R7. Don't forget all the wall framing surface area is counted at R1 and probably makes up about 15% of your wall space.

Depending on how much glass you have, More insulating is probably not going to help much. Newer windows can help a lot if you have older windows. Filling drafts will make a huge difference. New window installers will usually use spray foam in the gaps, so that also helps.

lol! I must be showing my true colours, but that actually sounds like fun!

Buddy of mine had the relief valve seize and his water heater. It literally exploded. Took out half the wall of the little closet it was it!

I check mine periodically now :)

I can't see how it would be an issue to unhook the inlet, just plumb in a shutoff valve on the inlet. It should be fine to keep the power on to keep the water hot over night. Just make sure the relief valve works... :)

Not sure if you considered this, but if you unhook the supply line, you will need to have a pump in the shed to actually use the hot water, or just gravity feed from the drain.