Correct, this will update until it becomes the full game. No additional purchases are required if you decide to buy it early.
As for the question about itch, I don't really know what sort of scenario you mean. Regardless from what I understand, you should always have access to purchases once made.
And for the release date, there's no particular date in mind. I'm currently working on finishing chapter 8 of the 12 main chapters. If I can complete each chapter in a month or so than roughly 5 or so months. But some things might go quicker and others longer as I've learned. The early Access will update every time I finish a new chapter however, until all content has been added.