Use lithium grease on garage door hinges. Lithium grease adheres well to metal and should perform better than WD-40 over time. Silicon based lubricants are good too, but I'm not sure they'll adhere to metal as well.

Do not use conventional WD-40, as it'll run off. The brand does make a lithium grease in a similar looking spray can, so be sure to check the label.

IMHO, i think you're on track. That being said....

Personally, when i have heavy cabinets to mount (garage), i like using a wide French cleat to support the weight. Not sure if it makes a huge difference, but i think about the cleat being mounted across the cabinet, as well as two lags per stud across the whole width, as means of distributing the weight. Is it necessary? Dunno, but i like to over build a bit, not knowing how well each lag is biting into a stud and not knowing exactly how the structure will be used long term.

That is what i was thinking. It's possible to estimate flow and pressure based on pump demand (current, etc), but you'd probably want to do so real testing and trending to understand the actual correlation. This is really the only shortcut around modeling every electrical and mechanical stage of the process, accounting for losses, variability, etc. Either way, it becomes a bit of a science project.

Really what you're trying to do is vary the motor speed and torque, not so much the current.

At that point, the best way to do this is with a VFD (though you'll need a rated motor). Variable Frequency Drives are sometimes called Variable Speed Drives for just that reason.

Technically it'll work However, a few code issues...

1. your dishwasher needs to be on a GFCI, so change that.

2. NEC actually allows disposals to not be on a GFCI, due to false trips. This suggests not sharing a circuit between the two. Now, multi wire branch circuit is common to feed a disposal and dishwasher, from two different poles at the panel and share an neutral. But this is done with 12/3 from the source -- something i suspey you already knew.

We just redid our kitchen, and had a MWBCn feeding our dishwasher, light over the sink, and disposal. I ended up running a new dedicated 12/2 service to the disposal, and glad i did.

Yeah, this is probably best.

Though, there's nothing dangerous about the ground wire just hanging out. It's bonded to neutral at the panel, as it a should be. The circuit downstream of the Romex is only two wire (hot and neutral). You don't want to tie neutral and ground together outside the main panel; that would be an NEC violation.

However, a fully proper install would bond that bare copper ground to the metal junction box in the picture. You don't need anything complicated, just grab a spare piece and join with a wirenut to extend, and land it on a screw in the box.

This is your opportunity to change the whole thing, upgrade as you wish, and/or put in something quieter or higher CFM output.

We bought our mid-90s house about a year and a half ago. I replaced all 5 fans (2 1/2 bath, toilet room, laundry room) with all new units and am glad i did.

This. The first question i would ask is "does the floor slope because the floor joists are compromised, or because they are sound but out of level?"

Also, how are the floors in the rest of the house? Does everything slope? This might be the difference between a bathroom floor leveling project, and a foundation project.