Nope. He can't just retcon the entire ending. Even if the spin off isn't writter by Kishimoto, it's still part of the Naruto universe canon. What's done is done
yes he can, other series have done it as people have said, and the anime could easily have another ending, heck they already contradicted the ending with the mecha naruto fillers and the road to ninja movie, you're telling me they can't have an alternate ending and multiple different spinoffs and universes? dragon ball seemed to have no problem with trying to retcon gt now did it?