[[WMG: Has Isaac come back to life somehow without our knowing about it, or did he simply write dozens of issues years in advance?]]
Because those are the only two explanations I can think of to explain why, long after Sylar killed off Isaac, his Ninth Wonders comic is still around and has up to date issues.
* Actually, in the show's timeline, only about six months have passed since Isaac's death. He died in early November 2006. Four months passed between the end of Season One and the start of Season Two, with the action starting up again in March 2007. Season Two took place over a couple of days in March 2007 with the action thus far taking up about two months, if Hiro's visions really did show One Year Ago (i.e. Six Months before the first episode). This means that Isaac has only been dead for - at most - six months - which is well within the advance time most professional comic artists work in... although it is probably nearing the end of his run.
* On the other hand, we don't very well know that Hiro and Ando are "in advance" at all. Hiro didn't really know what he was doing when he teleported them to the bowling alley and then the comic store, and we all know what he ''used'' to be like with teleportation. It's more than possible that he accidentally displaced the both of them a bit into the past. This wraps up the slightly-dated comic references (eg, complaining about ''Spider-Man'' revealing his identity instead of complaining about "One More Day"), as well as why there's a "new" ''Ninth Wonders'' when Isaac told the delivery guy that the unfinished one was the "last".
** You could also write it off as being part of the delay between new American comics being shipped to Japan.

[[WMG: Why does African!Isaac have the same artistic style as Isaac did?]]
Also, one of the murals seems to depict Parkman shooting ''Ted'', which never happened. The shot figure in the mural is wearing Ted's garish red checkered shirt. It doesn't seem to be Sylar (re: garish red checkered shirt), and it looks like a guy so it doesn't seem to be Claire either.
* In Peter's case, using the power also granted the same art style - even to the point of allowing him to seamlessly complete an unfinished painting. Could be that, to some degree, the style is an inherent part of precognitive painting; Sylar's style is different partly for stylistic simplicity, and possibly in-universe because he got such a handle on the power that he didn't need to use the "original" styling.
* Or they just didn't want to hire multiple artists and develop multiple styles for all the characters that can paint or draw the future.