There is almost no reason to buy card medals at the Mog shop. I've written longer posts on this in the past, but the main trouble is: because it's random, you can't ensure you get enough duplicates to HP the card unless you have a pile of medals large enough for Scrooge McDuck to swim in (well, at least 50). Keep in mind that an HP SR is stronger than an uncut SR+. Heck, they're stronger than most SR+s with three or sometimes even four limit cuts. Get medals through card recycling instead, by selling HP'd R+ and SR cards you no longer need.
If you want to spend your Mog Medals to get a card, it's usually best to buy every copy of the SR+ card on sale. Check the Wiki
to make sure it's good. The Merlin that's on sale right now is a Lux-focused card, and a good idea at your level, since it will help you ensure you get more Lux prizes every week. There's a special, longer Lux prize list next week, so all the more reason.
Depends on what you mean by "card shop." The Mog Shop changes every month. The Card Medal roster changes every three months and changed at the end of January. The Raise Card roster changes nearly every month, and so does the Kupo Draw, but those aren't rigidly scheduled. The Ticket and Mog Point rosters essentially never change.
If you're looking for Roxas, there's a hope he'll show up with other eligible bachelors during the White Day event in March or so. Save your Raise Tickets. Hard to say when we'll get the White Day cards: they might wait a month as usual, or they might roll over the moment Valentine's Day ends.
"Stable" is up to you. There are two major red flags about Guilt Missions for new players:
- Unless you've finished the story missions, you may not have unlocked every available Guilt Heartless. That isn't the case right now, but just a week ago, one of the Guilt Heartless was a Heartless that only shows up after you clear Dwarf Woodlands.
- The Guilt List isn't usually stocked with Heartless from the current event, so doing the Guilt missions is usually a distraction from all the other missions. A new player should be doing Story and Event missions, which return more stuff for less effort. This isn't the case for this event: the event Heartless are on the list, so you can do event missions and gather Guilt at the same time.
On the other hand, Guilt Missions are the only reliable way of getting Orichalcum, and without Orichalcum, you can't upgrade your Keyblades past Levels 10, 20 and 30. You're going to need to do them eventually, so it's up to you to judge when.
If you want my recommendation: I'd say you're probably ready to start harrying the bottom of the Guilt List for Orichalcums, and the Bitter Macaroons will give you event prizes and guilt. If you're willing to use Guilt Up Time to get all 700 Guilt (a huge number, but workable with the extra prizes from missions) you'll get an Orichalcum. You'll need 3 to get a Keyblade to Level 11, but a start is a start. Unfortunately, the Creeper Bouquets may be out of your reach, and the Chocolate Statues are out of everyone's
reach, but we like to pretend.
You should buy every AP Plus in the shop. You have the money for it: buy the ones with no timer (they won't regenerate) and the ones with a timer (they'll regenerate every month - keep on top of them). Check the mission list for every mission that has another AP Plus. Remember that AP Pluses have to be manually used. If possible, join a party with as many members as possible. Once that's
Potions are relatively plentiful. But you shouldn't use them to say, keep playing forever. On a normal day, it's better to use Potions for two major purposes: to keep fighting powerful monsters if you don't have enough AP to fight them period (this is risky), and to milk every minute of Guilt Up Time if you're in a hurry to complete the entire Guilt List (you shouldn't be since you only have to focus on the Macaroons, but that's up to you). Oh, and they're good if the game is about to go into maintenance and you need to do just one more thing
before an event ends forever. As you get stronger, you might come up with other special "rush" scenarios, but these are good guidelines.
Generally, you do wait for it to regenerate. You should get in the habit of waiting and playing the game multiple times a day, if you like it enough to do so. That's what the devs want, and it's not a bad way to play, either. I used to do twice or three times a day. Play as often as you feel comfortable and interested, but don't feel the need to use Potions to fuel yourself along.
Almost every Key Art that has ever been released has been for pay only (for pay or tied into a separate purchase, like a Japanese copy of 2.5 or V-JUMP magazine). If we're lucky, they might dangle another in front of our faces during the Second Anniversary like they did the First, but don't get your hopes up in the interim.
If you are the paying type, they're usually one a month. Two, this month.