True. I'll reach back out to him and a few others before I start working on anything. Thank you.

Thank you. I will give the structural engineer another call in a few weeks, hopefully he hasn't retired yet. He built his house in the 70's and had a similar issue (mine was built in '77), but hadn't addressed it at the time.

Adding the beams may be a better permanent fix without having to disturb the flooring. The majority of the bowing is in the hallway (dead center above the garage) and part of the primary bedroom. If that is rectified somewhat with beams across the garage I'd be satisfied until I explore other options after seeing how it holds up after an extended time. Not looking for perfect just better than what it is right now.

This works but decora switches usually have both screw terminals and push-in terminals.

If you wanted to get rid of the pigtail you could put 1 of the doubled up wires on the line screw terminal, and the other into the line push-in terminal. Then connect the single load wire to either of the load terminals.

Yerp. Sometimes still done today. It's not up to code as we need a neutral in the switch box.(Now) Black feed white switch loop. Neutral already existing at the light connected to another point in the same circuit (hopefully).

Edit: You can't always go off color of wires. I wire 3 and 4 way switches using white wire.(14-3 with ground) I identify with tape. Some electricians don't.

Any reason not to just use a small display?

Thank you! That is alot to think over. It would be a bit expensive but if it takes me 2-3 weeks and another $2500+ in Hardwood/Subfloor its worth it to me if strengthens the joists and lasts past the life of the replacement flooring.

I was referring to the linked article, not OP's picture. As I assumed you were doing, too, when you said, "Not those colors".

The wiring is line in/line out which is the two twisted lives (black) as well as a switched live out (black alone).

The whites are the neutrals.

Wiring a switch to bridge between live and neutral (as it would be if they copied the pictures) is a problem

Sounds pretty normal. For a single pole switch, line and load are interchangeable. Without a neutral in the box (actually I lied - there is a neutral in the box, but you can just ignore it; tuck it deep in the back of the box), there's nothing to confuse or do wrong. Ground to ground. One of the black lines to one of the gold-colored posts, the other black line to the other post (you'll want a pigtail for the line that is two twisted-together black wires). Differentiating line vs load only matters if you're using a smart switch or 3-way switch (two switches control one light).

Looks like a shittily wired box. Either there's a line and neutral with no load (in which case, you would wire a receptacle there, not a switch), or it's an old school no-neutral lights-only loop and they used white for load. Both are wrong, but given they're wiring a switch one would have to assume it's the latter and would still work even with the colors being wrong.

Switching line and neutral would be pretty stupid.

The dimmer switch that was here came wired from the original modular home builder, a little more than 20 years ago. There were 2 black unmarked wires coming from the back of the sealed dimmer switch, one wired to the single black wire in the box, and the other wired to the pair of black wires. There was no place to screw the ground wire, but it was inserted through a hole in a metal tab, and twisted to itself.

This is not for a 3-way switch.

IF (BIG IF) this was previously wired exactly right, the bare wire (ground) is properly wired. The two blacks SHOULD be power into the switch and power on to another device. The single black wire should be hot to the light's hot lead.

I'm concerned that maybe the previous person made this work but got the colors wrong, creating a shock hazard.

Need to make structural fix first. Otherwise leveling it is pointless - it will be out of level again quickly. Add the beams necessary to support, wait, and see what happens after it re-settles (6-12 months, at least).

Otherwise, again, you'll just be re-leveling it later.

How thick is self-leveler? unless it is thin enough that it can be easily scraped, that's gonna be a pain in the ass. Most SLC is very fine aggregate, and doesn't come out in chunks. It will also be insanely dusty I would rent a demo hammer, zipwall off the area, and use a lot of vacuum collection + full face respirator (not just half-mask).

Like that in layout, not like that in colours.

If you wire the switch like that it will go bang

That's constant power feeding another switch or plug the single wire the output to the light

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The engineer has the right idea, if the joists are sagging, the sag may get worse.

Right. Learn on somebody else's home. /s

Yes you can do that, just wire it back up the same way. Put the single black on the top screw, and add a pigtail to the two twisted wires (add a wire and marette to the two) then put the pigtail to the other screw. Easy peasy!

It likely provides unswitched power to lighting in the house.

It's wired to the new switch on the line side, and working fine.

Is that doubled up wire on the line, or load side of the switch?

There's no switched outlet in the room, so it's not that. Beyond that, I have no clue.

Call an electrician. Please don’t be learning on your home around when it comes to electrical.

The single black was most likely the light and the twisted pair was likely the branch line. The switch sits in the middle to be a switch/interrupt of the power.