Will Kelbo have to give up his powers for Justin, Alex or Max? It looked like Jerry had to give up his in the movie.
I don't believe so. I think the rule goes that there can be only one wizard per set of siblings. Jerry wasn't actually giving up his powers, he was merely producing powers – if that makes sense.
"No More Sugar, Sugar", where we find out Oliver is diabetic, never aired in the US. Is it canon?
Depends who you ask, honestly. After all, people outside the US will take it as canon. I don't know why it wasn't aired here, but since it was available for download, I think we should consider it canon.
When Jackson was "dating" Hannah, why didn't people at school pester him and/or Miley about it? For that matter, why don't they bring it up any more?
I believe that they did, but this action happened off screen. It wasn't integral to the plot. Likewise for why people don't bring it up – it's been established that Jackson is a friend of Hannah's, after all. I assume all this takes place off screen too.
In "Fashion Week" one of the models says that she's an 11th level Cleric, yet when playing D&G later she casts Magic Missile, a Wizard spell. How could she do this?
Assuming D&G is basically the same as D&D, there are two ways she could have achieved this. One is simple: She wasn't playing the Cleric character at the time, they had all started new characters for a campaign. The convoluted answer, which I thought up first, was that she's a Cleric with the Magic domain, and was casting the spell from a wand or scroll, because the Magic domain lets you use those as though you were a wizard (of caster level equal to one-half your levels of Cleric, I believe.)
Also in "Fashion Week", why did Alex use the wizard copy machine instead of, say, the Pictures from Faces spell?
My explanation is that the picture from faces spell would only make it a picture of the dress, and it would, as such, be paper, essentially.
What universe does JONAS fit into?
Ooh, tough question! You would think that it's not the universe proper because the Jonas brothers exist in that (From the Hannah Montana episode "Mr. Jonas, Mr. Jonas and Mr. Jonas", and in JONAS they're a band called JONAS, but made up of the Lucas brothers. However, my story is this: There are TWO bands of brothers that look exactly like the Jonas brothers – The Jonas Brothers themselves, and the Lucas Brothers in the band JONAS. With everyone already having plenty of doppelgangers, I think that this is actually not unlikely!