1, 2. Your priority should not be to have a party of every element but have 2 parties that can handle the majority of the content. You main party and a side party to deal with events that have elements your main is weak against. Unless you go light or dark, then focusing just one of them is fine.

3. Depends on what other eidolons you have. Apocalypse and Phoenix are 2 of the best event ediolons you can use as a main along side Garuda. Element attack is not the same as character attack, with element attack being much better end game.

4. That's completely fine if you need her to heal. Though if you get Brynhildr, replace her with her.

5. Use assault skill weapons. All of the same element of your main team. Buy a bunch of gem gacha and use the R weapons you get to enhance the skills for all your currently using weapons. Try to at least get them to Lv5 or higher each. Should not too difficult to do.