Hi, Most buffs are now self buff altho there are some party buffs like group almighty, and some rumir (new class) buffs. Forgot these names at the moment tho haha.

Yes, you can always buy the exp scrolls from the IM. Opps, edit. You can always buy exp scrolls and seals but with 400% seals are only sold in special sales.

With twins you need the set, like 2 (keena and Hart) of the same element. So if you want Earth twins (Vit), you will need a Keena gem and a Hart gem to make the set. And you can use 2 different Elements on your char. Like Earth Twins + Metal Twins (just pick your preference) And sometimes it's hard to get a set so you have to keep trying. And I'm pretty sure they don't have a lvl req.

and for 4, yes. Altho some skills have a lvl req before you can add more points, do be aware of that.