The dmg, hitboxes, proration, health etc will all be altered in the main version, so don't be frigthened by it, all characters have to come out before they can patch and start buffing/nerfing balance.
This game is far different from games like 10.5/12.3 and can't really be compared to it.
There are a total of 8 slots where you can place different skill cards/items/spellcards, and each card can be selected several times. Each card may or may not act differently depending on what slot you put it in (The skill might do an extra attack/bullet, might be angled differently, or simply just do the default skill)
Spellcards sort of works like IaMP, you can declare a spellcard when your lifebar is fully blue, and then use it again to activate it. If you have 100% popularity, you can do your mega spellcard (everyone has 1 of those).
Also each character seem to have a different system to themselves.