You should post this question to the Terry Love plumbing forum, they're great for stuff like this, the users there have lots and lots of plumbing expertise.

I suspect that they would tell you most of the stuff you're proposing will not help your situation and you will probably be worse off doing it.

Some tankless water heaters have built in recirc pumps or have an option for an external pump. You do need insulated piping though.

Eww, I don't want snails falling into my escargot!

This is not a fan rated box. A fan rated box would have 10-24 screws and screw posts or double ears. https://imgur.com/ldUwkw1.jpg

Don't just cross your fingers on this, you can get a fan rated pancake or saddle box and they're not very expensive.

Yes, though colors don't matter too much. Keep in mind the switch is just making/breaking the connection of the hot wire, so the reds are hot before the switch (line - always live) and the yellow is hot after the switch (load - switched on and off). It's best to double check with a multimeter but that's what it looks like from your pictures.

At the fan box white is neutral, yellow is hot, connect a ground wire to the metal box. At the switch the reds are line (one incoming hot, one going out to feed another switch or outlet) and the yellow is the load (hot at the fan box). Again, ground the dimmer switch to the metal box.

A regular drill will easily drill a ½" hole in a concrete block, hammer drill isn't terribly necessary. Yes on the masonry bit though.

If your house is conditioned you're going to lose a lot of heated/cooled air by doing this also

With that short of a span anything from 3/4" mdf and stronger will work fine.

Til what a peeler core is

Now you know, and knowing is half the battle!

You can buy adhesive match striker stickers.

Example: 0.5"x2" Match Striker Stickers Strike Paper Adhesive Pre-Cut Matches Lighting Strip (50 Pieces, Honeycomb) https://www.amazon.com/dp/B0B4Z2M9ZF

Clamp a piece of wood on either side of it, this will prevent the outer layer from getting chewed up. Then use whatever metal-cutting drill bit you need.

That's actually what I would do, just shave down the parts over the mounting screws that the bracket will hit. Leave the rest of the flange obviously because it holds the box to the drywall.

Old work boxes don't care about drywall thickness.

Cut the horizontal pipe and use a fernco coupler to connect it back together when you're done.

Get all that silt out of the pit while you're at it, that's most likely to kill your pump.
I agree that if your pump is still working there's no need to replace it right now.

You can cut plastic pipe just with a sturdy piece of string.

https://youtu.be/MC3S7OhfSk8

The felt strip is meant to go on the top, it's to distribute the water evenly across the panel. You installed the panel upside-down.

https://www.diypartsusa.com/product/318518-761-bryant-carrier-humidifier-water-panel/
>The 318518-761 humidifier water panel includes an integral aluminum frame and felt wick on top for even distribution of water through the media.

That would be perfect. This type would also work.

Your old box is probably nailed into the ceiling joist, like this box, so expect that it might be a bit difficult to remove. I've sometimes had to use an oscillating tool to chop it to bits in order to remove them before.

I used 6" schedule 40 pvc for my spud gun and it handles 80psi fine. It's rated for 180psi.

"No excuse! In fact, you need to come in early today"

Good luck getting 20' out of two 10' pieces with a lap joint

For anything up to maybe a 3/16" bolt I wouldn't bother buying a hammer drill if you've already got a regular drill.

Advocating for all the quality of life stuff that every other first world country already enjoys

As long as the pipe looks to be in good shape, not too rusted and no leaks, there's no need to replace it. I'd probably put in a new check valve just because they're cheap and having been dry for so long the old one might not seal very well.