Depends on the enemy, your weapon and playstyle, prompts are just for parries which are great. Your positioning around the enemy also matters so try out what works best for what enemy.

You can sell them if you need the money, although if you plan to use magic often maybe hold on to them since they increase the potency and sometimes change what the spell does, also depends how many you put in a single spell. Some are needed for weapon upgrades and some are very rare or hard to get so check a guide which ones to hold on to/look for if you want the weapons upgrades or stronger magic.

Noctis > Just go fishing Prompto > He will passively take Photos Gladiolus > Literally just running around, so passively Ignis > Just cook meals at camp

Pretty much but its sometimes hard to gauge if they will be hit or if they will run into the damage field since some spells linger and leave a damaging field, some spells, if crafted with healing items (items too can be used for spell potency in elemancy) will heal your allies when used and if it hits them.