Get a stronger stud finder. My walls are drywall then plaster. Almost 2” thick in spots. A regular stud finder does nothing for me, but I’ve got a higher end one that can find them.

Otherwise the old outlet box trick is a good way to start.

Measuring 16” from the corner isn’t a good way because the wall probably isn’t a perfect multiple of 16”. That corner could have been the end of the wall when they framed it. A 170” wall from the tail end is going to have studs at 10”, 26”, 42”, etc

Most door bell wiring is very low voltage (picture speaker wire or coxial cable). As long as you’re not running 110v through it (some do and those are not compatible with ring doorbells) you’re fine. You can use pretty much anything.

Just put gravel down. Might want to use pallets or a couple of boards under the firewood stacks just to prevent ground contact.

Even cellulose (basically paper fibers) is bad to breathe in. Mineral wool (which is what it looks like) isn’t good for the lungs. A little here and there isn’t that big of a deal, but I wouldn’t make a habit of breathing it in. Especially for kids with developing lungs.

I’d do a drop ceiling. Pretty easy to do and not very expensive. It’ll also add a little more sound dampening.

If you really want to go cheap and easy, you can staple some plastic sheeting to the ceiling instead. It’ll serve it’s purpose but be pretty ugly. Hang it on the walls too, if you really want to get a murder room look.

Code requires the bottom plate to be PT, so assuming it was done right and those walls weren’t built super long ago, it should be fine.

Do you have access from below? If so, I’d just run a new pipe from the basement to a new valve and supply line. The valve in the toilet tank could also have a lot of build up in it.

Also, get a water softener to prevent this issue from continuing.

Those springs are very easy and safe to do. When the door is open, there’s no tension. I did mine for $18 (at Home Depot). I don’t have any tips for you, but you can do it!

Wait, I’ve got one tip. Do both of them. Mine broke, so I had to replace one, but I was advised to replace them both at the same time.