1. You can do most (all ?) content if you put enough time playing the game and farming things.
2. The one you'll get 100% eido for. But honestly, you can't really plan your team around one element, you have to do with whatever gacha will drop for you.
3. Get SSR event eidolons, get SSR event weapons with Assault/Pride, get raid SR weapons, join decent/good union.
TBH, you'll be quite behind other players in every aspect of the game (kamihime, weapons, eidolons), so do not be surprised when for first few... months people will be much better than you.