1. I consider cards lower than 5 low cards.
2. Don't reset the save for moogle rooms, it's not worth the time. Just sell low cards and use the points to get new packs later on.
3. I usually set up decks for sleights, I find the best setup is premium, non-premium, non-premium. The premium cards will dissapear anyways, so they might as well be first in the sleight. Then when you refresh, you're left with non-premium cards. The big thing about this game is few enemies have 0's, so as long as you use sleights you take no damage except for bosses. As far as boss specific decks, just make one for that specific boss if you're having a hard time.
4. I usually use stuff like blizzard raid for the first run through, then have the attack cards make their own sleights for the second one. You'll rarely need to plan a third run through, because they'll be near if not already dead.
5. Recovery is only useful if your cards get broken. With sleights that's rarely the case, so it's usually not helpful.
6. No, you just get gud. In all seriousness, with the auto aim mechanics of KH games, you rarely miss an attack.
7. Absolutely. Summons aren't particularly useful. In most KH games they provide brief invulnerability or other forms of cc. But in RE:COM, you have that with literally any card that doesn't get broken, and summon cards last longer and have a higher chance of being broken because of it.
8. Magic is op in every KH game. Just don't use fire on axel, thunder on larxene, etc. And you're fine.
9. Just a couple really. It's mostly just a quick reload, although some restore cards lost by sleights.
10. I would say 0. They're the most expensive card, and less useful then a high valued sleight which will break your opponent's card anyways, and be difficult to break during your attack. It's more gimmicky than anything really.
11. A 0 summon, item, or spell lasts longer than a 0 attack, and will almost certainly be broken before it goes through. So attack cards, being quicker, are the most effective 0's.
12. First run through, 009,009,009... All Ars arcanum. Second run through 090, 909... Every other ars arcanum. Third run through, 900, 990, 099, 009... Every other Ars Arcanum. Fourth run through, 009, 099, 090, 090, 990, 900, 909, 909... 4/8 Ars Arcanum, but not every other. Notice the first run through, you waste no 9's, only 0's. Every one after, it's 50/50, although only half after the first reload are ars arcanum. With 3's every sleight wastes a card of equal value, while 0's are considered less useful than 9's. With a deck of all 3's it's easy for bosses to have a sleight over 9 in value, which you can't break. With 0's and 9's you can break their sleight with a 0, 9, 18, or 27, and all the cards you need are close at hand, so you can do it at any time. Since after the first sleight, you always have as many 9's as 0's, you always have this option until you run out of cards. But you can elixer up and start over fresh.
13. No, in this game the best defense is a good offense. If they can't break your sleights, they can never attack, and if they never attack, they can never damage you.
As far as decks go, it's been a while since I've played. But beginning decks should largely be attacks with a cure and a couple potions. Early spells are fire and blizzard, which heal common enemies in early worlds, so attacks are usually better. Besides, without many sleight commands, you just need to spam normal combos anyways.
Mid game, start throwing in every type of strike raid you can, these deal pretty decent damage, can hit many opponents with good positioning, and leave behind two keyblade cards when reloading. With a bit of planning, these can be sliding dash, stun impact, and other sleights to make battles easier.
Late game depends largely on what cards you've accumulated and what boss you're facing. Raids will be enough to deal with the encounters, but bosses have quicker and more damaging attacks at higher values that are hard to break without a handy 0. This is why 009 ars Arcanum decks are useful late game, you can quickly break any sleight then counter with your own. Since the AI spams single cards more than sleights, only a 9 or 0 will break ars arcanum, with the exception of the rarer used sleights, easily countered with a quick 0 or 27. Aside from ars arcanum, there are a variety of useful sleights that you can consider.