Character Card System

I'm also fairly sure that the card bonuses don't interact that way between the merged worlds. Could somebody confirm that there's no point to stacking a bunch of A sets on merged worlds?

Just made an account to report my findings with the card system. It's quite lengthy so have a spoiler.

 Spoiler

My main server is in Mardia. My two decks are

Deck 1: Corsair, Cannon Shooter, Jett (all S rank)
Deck 2: Mercedes, Demon Slayer, Marksman (all S rank)

In Yellonde, I have a lv 32 Cleric. I do not have any cards loaded into a deck on this server. Upon logging into it, it got the added stats from Mardia (+1400 max HP/MP, +7 all stat, +3% crit rate). Creating a new character on the server also gave the same effect. Curious, I added the Cleric's card to the deck. I got back on my two characters only to find that the added stats were gone. I removed the Cleric's card from the deck, logged back on, and the stats remained the same.

So now I create another new character in Yellonde, making sure there are no cards in Yellonde's deck(s) but keeping Mardia's. The character had the Mardia deck bonuses applied to it. I went back to Mardia, removed all the cards from my deck, and returned to the newly created Yellonde character. To my surprise, the character retained the stats. Even changing the Mardia deck around (using other characters I have), I could not alter the Yellonde character's stats--it remained the same. I throw the Yellonde Cleric card into the deck, and (no big surprise) the added stats disappeared.

Conclusion: Server A can provide its Card effects to Server B provided that Server B does not have any cards in its deck. The simple act of adding a card in Server B's deck will cancel the bonuses provided by Server A; removing the card will not bring back Server A's effects. You can, however, create a new character in Server B and get Server A's effects as long as you have no cards loaded in Server B at the time you create the character.

I forgot to test if Server C can have the deck effects of both Server A and Server B (of course, as long as Server C has no cards loaded). I want to say yes.

Based on his work, you might be able to get 4 worlds or up to 8 sets to work but at a cost of losing your main's (home server's) decks.