[quote=@The book of bad juju]
Ooh, neat. Nice little chance to expand on my character's backstory here, so lets begin.

1) If number 39 isn't available to anyone, how is it explained in-universe why a card people have never seen used and presumably never heard of is the figurehead of a whole House, representing the whole branch of XYZ summoning by itself?[/quote]

It's not expected that the characters understand what the cards are, or if they're really significant. The houses were named more for player/audience recognition. In-universe though, it's existence is known, as in it's in the wiki and people have heard of it/there's been speculation about it, but it's unavailable and no one outside of Industrial Illusions has any idea of how to get it or why it was never printed.

[quote]2) What is the canon status of the Rank-up Magic cards? Do things like Astral force or The seventh one exist at all,  given that all of the monsters it supports are one of a kind and presumably their Ranked up variants as well? Probably just the Phantom Knight and Raidraptor support exist right now, but i'd just like to make sure of their canonicity[/quote]

RUMs are still a thing and they still work. Because nobody has over-hundred-numbers though, nobody has a reason to be running the Seventh One. Whether it exists or not is a non-issue.

[quote]3) When you mention that Numbers cards still have a bit of power inside them, do people know this? Is this an accepted fact of the universe, that sometimes people playing a bunch of unconnected cards will suddenly act differently and go a little bit kooky?[/quote]
It's been on the news. Some people take it as superstition, others do not. 

[quote]4) Does Exodius count as an Exodia card, or is it more common then that?
[/quote]
Still rare in that you'd have to pay money for it.