This won't work if the fill valve has failed, and it's pretty common for that to happen on these. Sometimes it just needs to be cleaned out real quick, but often it just makes more sense to replace it.

A pro is worth the money here, even if just for diagnosis/consultation. They know the area, the building materials, the climate, etc. Very unlikely this is an unusual problem in your area.

Seems likely that it's a foundation/drainage issue. There's no easy fix for something like that, so it's well worth some money to figure it out definitely. To do otherwise is pennywise pound-foolish.

And make sure it's the quarter-turn kind.

The lines are mill marks. They were already there, you just didn't notice. If you sand enough, they'll go away.

The inspector is looking for defects, not the absence of features you might like.

It's not.

You think it is because you don't understand a thing about what you're getting into. I'm not trying to be mean or whatever; that's the simple truth.

You can paint over the finish that's currently on there. Scuff sand and paint.

If you want to use stain, you need to remove the clear finish first. Sand, stain, clear. Wipe-on poly is a good beginner clear coat.

> very hard time fitting up there to do any manual work

You still want to get up in there to seal any air gaps. It's a lot of work, but if you don't do this, all the insulation in the world will only get you so far.

Blown in is a good option, but sealing is very important.