Hot water can hold more salt ions than cold water, as you can observe when crystals fall out of a saturated solution while it cools down.

This implies that it takes energy to dissolve the salt crystals.

So, when you add salt to water, it will cool down as it dissolves the salt, making the claim that the system stays cool for longer plausible.

Photons, mostly in the infrared energy range. Photons make up the vast majority of what we can actually "see" from the universe outside of the earth/moon system, though since recently we can measure some gravity waves too.