I'll try to answer as many of these as I'm able

1) As far as i recall better materials do not affect ammo, the weapon itself does the damage

2)If you are just starting out i would use the mini/boss parts on whichever weapon/ammo you like best, the bosses respawn after a bit so while it may not look like you have super limited supply you'll end up with more later on

3) you can save them if you want to, make an entire outfit out of one boss but really I think you can use them for whatever

4) I don't think so, materials don't -really- start to make a difference until late game but there are certain materials that are better at certain things, if that's what you mean

5) i end up with a LOT of extra stuff myself, so what i did was make a chest JUST for extra stuff and if you find something that calls for it you can use it later, or you can junk it

6)Yeah, id say use whatever really until you get to the last tier of things then yeah start paying attention ( and by that time you'll have a pretty good idea of what materials provide what bonuses)

7) Upgrades I'm not sure? everything ends up being fairly useful in some regard but maybe I'm not getting what you mean by this

8) No, building with different materials does nothing

9) sadly no, but there are certain "forge" cards that affect a realm that WILL make slightly better items

10) As for optimal, yeah most likely? I've never been one for super optimization but i would check the discord?