THEORY: The Dragon Deck that Adam had is the Black Dragon deck. Corrupted by Xaviax of course.

As to Kit's father appearing in the mirror... I guess all of that depends on whether or not General Xaviax's ability to change shape goes beyond the two forms we've already directly seen.

As to why Dragon Knight's Advent Deck doesn't have a contract, that something I've wondered as well. Maybe just to sell a "Blank Knight" toy. Maybe Dragon Knight is unique amongst other riders (even Onyx?). And why is it that Xaviax orders both Torque and Strike not to vent Dragon Knight? Maybe someone who has seen Ryuki knows, but I don't. Unless Xaviax thinks that once Wing Knight is out of the picture Kit will just give up and join him. But then why does Strike say "I'll save you for later"?

He wants Wing Knight out of the picture because he is the bigger threat. Dragon Knight is a nobody he can take his leisure with him as he sees fit.

I think its more then that. Xaviax seems to have a plan for Kit. And is perhaps decieving him somehow. Dragon Knight is key to his plan. Just like Adam was before Kit. This should be come aparent down the line.