I think it's unrelated. Remember that the Disney-Marvel buy out happened after the show's second season and in fact is the reason why the show got canned. Weisman said that he learned about this in the news and wasn't told so in person. Weisman said that he was going to age up the characters as the seasons progressed but he was never able to do so. But such is life.
Disney/Marvel focusing on high school Peter has to do with their first cartoon series being a version of Ultimate Spider-Man (that had nothing to do with the comic other than the name). A fairly bad cartoon with decent quality animation FWIW and for them targeting a very small and very young audience. Weisman's Spectacular for instance was written with greater maturity and targeted all ages, like the DCAU.
This was done before to some extent in Raimi's Spider-Man 1. If you watch that movie the origins of Goblin and Spider-Man are crosscut with one another, running parallel but not intersecting until the final act. Norman would have been Goblin without Peter, and vice-versa.This show also does an interesting idea with the Green Goblin I haven't seen before: The Green Goblin is an evolving character like Spider-Man.
Spectacular Spider-Man's take on Norman Osborn is probably the most compelling and convincing version. You get right away why this guy is the biggest villain in the rogues, and he's charismatic and compelling as both Norman and Goblin...and the show goes back to the Ditko version which always portrayed Norman as sane and rational and entirely in control. As opposed to the "Goblin made me do it" melodrama that came with Romita.That is how he goes from being an unknown to taking over a whole empire by the end. I wish the comics would put more emphasis on this. It would explain how Norman also becomes an Avengers villain by the time Peter gets to be a 28-year old high school teacher and an Avenger.