Now that I think about it if Kinzo was the author a lot of things would make sense.
The message bottles for one thing. If they were prepared before hand nobody really needs to find them in the ocean if Kinzo is involved. He just has to set something up with some publisher or some friend of his to release the "message bottles" after he is presumed dead or when the accident happens. So even though there were sailors who said they found them that could just be a lie. And Kinzo does make bluffs and weird stories up.
Secondly the people who are framed would make more sense. The author seems to frame women most of the time (except for the one time in episode 4 where Kinzo is framed, which is the only exception) and Kinzo is portrayed to be perpetuating misogyny and teaching it to his Kids. He's also said to be a lot like Battler so the perverted stuff in the meta world wouldn't be out of the ordinary if he wrote it.
Third these stories made an obscure hobby like the occult popular after they were found. Kinzo says from the very beginning that he doesn't want to leave anything behind. He says he began with nothing and he'll die with nothing. However at the same time witch hunt finds rare books from his personal library, and they are formed after the Umineko stories circulate. Kinzo might have a motive to write these stories to make the occult popular.
Kinzo suddenly seems all the more awesome now!
EDIT: An important thing to note is I don't beleive anything I just said. it'd be awesome if it were true for episodes 1 and 2 at least. I just thought it was funny how that could easily explain how they were written if episodes 1 and 2 were written previous to the Rokkenjima accident.