Ny way to “Salt” unsalted cashew pieces

It is true, in my experience. However, if your recipe does not require salt, you should not be adding it, of course. Removing moisture from onion with the aid of salt helps it reach its desired state (brown or translucent, as desired) faster.

I am most familiar with the use of onion in Indian cooking. I generally add a pinch or two of salt to onion when I start sauteing it. Onion starts sweating as soon as you add salt and start stirring.

The reason that salt draws out moisture from onion is osmotic pressure. The surface of the onion acts as a semipermeable membrane barrier. Since the solution outside the onion is more concentrated with respect to salt, water moves out of the onion to the general liquid side, to balance the osmotic pressure on either side of the onion surface.

Depending on brand, it is approximately 1 1/4 tsp per pound (US), or a little more than 1/4 tsp per stick (4 oz).

For most applications, yes it is fine to substitute and adjust; you can just adjust the "salt to taste" step of your recipe in many cases.

There are a very few uses (such as yeast raised dough) where you want to be more precise. I would not use salted butter for a yeast dough by preference, but if I had to, I would calculate the amount of salt to remove from the other ingredients based on this ratio:

1 1/4 tsp salt / 16 oz butter