The issue with the writing is it doesn't seem very consistent overall, so we're left with no obvious guideline from Ryukishi:
- Ep1 says the story told of the events, so it must be at least a summary that an average person not associated with the story (a cop, a Witch Hunter) could read and grasp the narrative.
- Ep4 flat-out describes the amount of writing as "massive," as Jan-Poo notes. "Massive" is a strange word to use, and even if it's mistranslated, Ootsuki surely must've meant "a lot of writing," more than he would have expected from a message in a bottle (which, in traditional imagining, is usually like one page or so).
- Yet other sources (manga, anime, etc.) portray the bottle as fairly small and the number of pages as fairly limited. Even taking the novel's claim that it was crammed onto every page, you can only have so much there.
- There's physical limits. Jan-Poo discussed how time-consuming it is to write, and even if you have a great idea hand writing something takes more time than typing it. If parts were written at later dates or times, surely someone like Ootsuki would've noticed that and commented on it.
- This is silly too, but it's relevant: It's a message in a bottle. I've seen some large bottles, but the average bottle a person is going to have access to is going to be a medicine bottle or a wine bottle or a soda bottle. These cannot hold large amounts of anything, let alone paper. While much larger bottles do exist in the world, you'd pretty much have to track one (or more!) down quite deliberately to serve as your message-carrier if you had enough pages in there. And even then, you'd have to make sure the pages aren't so heavy that the buoyancy of the bottle is lost.
I don't know if we'd ever get a straight answer that satisfies all of these problems.