Adding sufficient heat would harm or kill a living thing.

follow the directions on the bottle. If it says fill up, then fill up.

3 are sufficient. Adding more will just create atmospheric drag and slow the rocket down.

In making a potion; after adding the prescribed ingredients in the prescribed amounts, add enough bulk ingredient (usually water) to make the specified amount. e.g. to five grams of NaCl add enough (quantity sufficient qs) water to make 100 ml.

To the best of my knowledge there should be sufficient iodine in the sea salt you use to make the sea water without adding more.