I heard that YAZAN retired around the time the ranked reward of that dwarfstar dragon was leaked. And we never see XOF anymore. I don't think there are a ton of dataminers now. The leaked news page takes longer to update nowadays.
Beside that, at this point even if with disappointment I would dare to say that the developers have no intention of making him unlockable. They teased a lot, but I don't think they actually know what to do w/ the character.
It is possible that they just put some characters into the files without necessarily a plan to release them and just to sort of have them on hand depending on how they want to develop the game. But I wonder what went wrong here? Konami is basically a silent company, so we have no real way of knowing about a lot of their choices.
My guess is that with Yami Marik being released first, with both of his ace cards available (Lava Golem and Winged Dragon of Ra) both of the Ishtar siblings already there, and almost all of the tombkeepers' cards available, they just did not know with what excuse introduce him in the game. I think what you said is completely right.
Sounds about right. Another problem would probably be the fact that they are now determined to not give characters decent level up and drop rewards anymore. And this is pretty much true for events as well. Any gravekeeper card would be considered as too "good" or fun. The same thing could be happening with Atticus if the red eyes structure deck is anything. Now they don't know what to give them.
Cards aren't an issue (we have enough Gravekeeper's cards not in DL yet that he can get as drops), but yeah, I think they have trouble thinking up an excuse for the event.
After all, card drop isn't the only thing in an event, they also need a plot for the event, an in-universe reason why he shows up.
True. We would need some kind of theme besides "such and such comes back" for an event. But what about stuff like bingo missions? I really wish they hadn't just turned that into tour guide from the underworld only now. Or was this the real purpose for it I wonder?