How much is a “round” of butter? This is in an old pound cake recipe

The weight of flour varies immensely depending on how densely it's packed and the humidity. If you've started with a weight-based recipe, I'd try to find a scale.

If that's absolutely not an option, I tend to approximate 100g = 1 cup.

Be warned, I've found some recipes that are really sensitive to the amount of flour and the difference between a lightly sifted cup and a scooped cup can really throw off the results. You might do better to try to divide another kg package of flour evenly and use the approximation that 0.6 lb is about 0.25 kg (i.e. one quarter of the bag).

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