Channel locks for pipes or even a pipe wrench

Totally would work if the neutral is switched, but it's not safe. It means the fixture is always "hot". In the end, it's not likely to cause problems, but it's backwards.

Really this boils down to your switch. Does the white or black wire run to it? Unless it's a smart switch, it will only have one, not both.

As far as adding a on to that circuit, you can tap into whatever is down stream (ie a light) and run a new line (white/black/ground) from that light to your outlet.

You can do the same from the switch itself also, but that's more of a fauxpau

Yes, you can insulate it. Rockwool is ideal, as it's flame resistant, while fiberglass/ cellulose bats are not so much.

I wouldn't vapor barrier. Ideally you'd drywall after the insulation, but that's also not necessary. Drywalling will however act as a blocker to airflow as well as adding sound deadening.

Returns are ducted, they just aren't insulated.

It's not uncommon to have one or two returns, but not one per room. You only see multiple in larger homes, as the return also directs air flow throughout the house.

I would not want a return in a wall cavity. Returns are typically where the filters are installed, although it's also not uncommon for that filter to be next to the unit, but you are trying to pull air from a cavity that has limited capacity. The whole idea of Returns is to allow for enouhh cfm back to the unit, you'd never restrict a return like this.

You wouldn't ever have back flow with a washer. Back flow occurs when you loose water pressure in the line, both of these lines would be pressurized by the main water, and would potentially back flow only if you shut off/lost pressure on main.

If anything you're more likely to get back flow from a fridge.

The fill valve is also well above any standing water (notice it flows from the top on washers). And even if that really was a concern you could just put a check valve in line with the washer hose.

Anything meant for stucco/masonry should be fine, although in theory cement basement walls should have an exterior sealant, so it really shouldn't be a worry.

Make sure you prime before paint.

I thought that was the 3.8L?

You had the 4L then, which is a very dependable motor. It's gutless and leaks oil, but dependable