Spoiler free slightly advanced questions about 1st chapter

I'm about 3/4 through the prologue and I love it but I have a few questions!

-I know that party members are temporary and I should prioritize orbment upgrades on Estelle and Joshua. But when a party member is replaced, is the gear and quartz I give that party member lost, or is it returned to my inventory?

-When should I prioritize field battles vs. command battles? Should I mostly just fight the smaller enemies in the field, and the larger ones in the turn based system? When should I use Craft Attacks vs. regular attacks? I also know that there's a way to influence the turn order through cycling through the battle types, but I'm not sure if I want to do that.

-When should I be using Overdrive, S-Crafts, Crafts, and Arts? Recently I've been using Overdrive in my second turn (especially as Estelle) when I use my buffs. I've also found the trick of performing your turn, and then using an S-Craft as a second turn, especially to steal an AT bonus. I'm confused when to prioritize using Crafts over Arts in battle, though.

-Are there any items I should hold on to that would be better used later in the game? For example, I have a few G-Pom Fruits and Zeram Powders. I've been selling my Powders for Mira, but I'm not sure if I should also be saving a few for when I get in a pinch.

-Is there any way to exchange an abundance of Sepith for another element type? And what should I be using with my Sepith Mass?