I question the salt in boiling water one. Sure it probably doesn't change the temperature or energy required but shouldn't it aid in nucleation to make the boil more bubbly?
Or are the existing impurities in most tap water such that salt doesn't add anything meaningful?
Salted water boils at a higher temp vs. unsalted water - this is known as boiling point elevation. But I don't know offhand (there's a formula) how much salt would be required to meaningfully raise the boiling point.
If you throw it in just before it's boiling yes, you will see the water bubble up for a moment for the reason you said...
once dissolved if anything at those amounts, it raises the boiling temp a little