I don't believe Great Stuff is waterproof; dab of paintable caulk on the hole

You don't sound like a do-it-yourselfer. Best bet is to keep looking for a plumber, or a *reliable* handyman (church?) to troubleshoot and fix or re-plumb

Add a couple of elbows & straight lengths so the relief drains outside the Skirting.

Secure power, secure cold inlet, secure hot outlet (not usually a shutoff here, but shut it if there is), open hot and cold faucets inside house to depressurize the lines. NOW, why couldn't you disconnect the water lines at the tank top /side? Then spin that sucker around and get a hose on the drain...

It just jacks back in - check you tube for a how to

Ask your building dept. I know that in Pettis County, regs outside of city limits are non-existent. Inside, a building permit would be required, which means inspections. No big deal, just use a reputable installer. But ask.

Yes your plan seems sensible. The hard part is removal, and then leak location & repair. Installing new ceiling is relatively easy, as long as all damage inside is repaired. You may find that the paneling gussets have deteriorated, causing the ceiling-attachment edge of a truss to sag. New plywo...

What lurks beneath those "thin strips of wood"... mwuuhahaha

Old will not come down easily; they are stapled to the trusses with a million staples. Flat-top screwdriver to pry, and channel locks to lever-pull.

Agreed. I'd bet a doughnut that you were "advised" by a contractor.

Hey subguy, me too. Only 8 yrs, was a nuke.

Didn't getit: [email protected]

Cheapest new windows is custom order from lowes; I get the nailing fins, we call 'em Nascar windows - think of a pit crew changing wheels. Butyl tape on the back of the fin, new 1-1/4 inch long, 1/4" hex drive stainless fasteners, easy peasy. Hard part is trimming out inside, and that ain't tha...

Combine them: 1 big window

Polybutylene... and it sucks. Often breaks at clamps or fittings. I use a pex transition fitting, but unless you have the tools, go with the sharkbite, say I.