1) Basically, yeah - the characters on the grid would be the combatants/cards.
Alternatively, you can naturally animate this however you want :)
2) Not really, when basing it off the Phase battle system, you can use as many combatants/cards in your phase as you want (same for the enemy). You'd need to control when to end the phase anyway, e.g. in the schematic animating the selected action.
3) Hm, that might be difficulty. An easier approach would be to just implement your own card system, where each card e.g. uses other items or abilities for their actions.
You can add your custom settings, sections or sub-sections to the editor - ORK itself is implemented that way, since it's an extension for the Makinom editor. There's currently no documentation for that, but it'll come in the future (scripting with ORK/Makinom is somewhat of a niche topic, since most don't use it).
4) That's hard to say and mainly depends on the details.
I'd say, most should be doable with schematics and extending the functionality - changing the source code shouldn't really be needed, unless you need to change something that can't be extended. E.g. adding a custom battle system doesn't require changing the source code, you can simply add your scripts to Unity and they're available in ORK.
5) Naturally, you can also do that - e.g. write your whole custom battle system not based on ORK, UI, etc. and just use ORK for the status system, item/ability setup for calculations, etc.
If you're more of a programmer or already have a somewhat working system, it might even be the easier approach for you.
E.g. using the combatants and items/abilities for the action calculations only requires a few lines of code.
6) Would probably also be possible, though I'm not sure how the card draw could work here ... unless you constantly switch out abilities (e.g. 3 abilities form a 'card' category, so you'd have to add/remove them as needed).
ORK can add abilities as temporary abilities, which makes it easier to remove them in bulk (e.g. just removing all temporary abilities after a battle) - though you can also keep track of this via (global) selected data, where you store the items/abilities of a card in.
As for the system breakdown, yeah, that'll need some custom work - especially the attack preview. While ORK can preview status changes for actions (e.g. how much damage it'll make), previewing move patterns or cells like that isn't possible out of the box :)