Best moment to put salt in spaghetti?
Salt should be put before putting the spaghetti (or any other type of pasta for that matters) in the water.

For 200g spaghetti (2 people) count ~2-3 liters of water and 20-30g rock salt.
You can reduce the amount of salt if the sauce you are using is already quite salty.

As a note to your point 2, the salt INCREASES the boiling point of water (a process known as boiling point elevation)

However, the increase in boiling point when adding 20g of salt to 2l of water is practically insignificant.