Each + is 3 Atk and 1 HP more stats. These are moved if you use that Eidolon to Enhance another. So combining one +2 with another +2 gives you a +4. These go up to +99 per item.
You gain a few +1 weapons and Eidolons from Gem Gacha every day, a fair bit from raid Gachas (raid tickets) and tons from Premium gacha.It's a random drop, just like potions. This random drop reduces to summon timer on your primary Eidolon by 5 turns.
You'd much rather have a potion, really.What do you know about the primary weapon?
Ultimately, your goal is to have max skill level Assault in every slot of the same element - SR or higher. You can get those from Disaster raids, but event weapons are generally better. Your Grid - leveled up skill levels equipped - is THE most important thing when it comes to your damage output.
Order only matters for Relic weapons, as those are special. Their skill only activates when they're in the main slot, and only for the matching Soul.