As for your second question, they make a basin wrench to remove that nut.
https://www.lowes.com/pd/Kobalt-Basi...B&gclsrc=aw.ds
As for the first, I can't see the 3/8 side of the valve in the pictures. In theory, it should be correct, but not 100% sure. I suffer from the same plumbing problems you do. Every job requires at least 2 trips to the store for parts.