Although I have read the author's page several times, there are discussions in several forum threads that are relevant that I cannot locate. Two examples are the preferred format for submission and the steps for inserting a Bookapy ad at the end of chapter. I saved the preview feature link somewhere, but I cannot find that bit of info either at the moment. Worthwhile? Doable?
An actual wiki is not worthwhile. It's prone to abuse (if anybody can edit it) and it's usually heavy on the servers.
Whenever a question gets asked a couple of times, I add it to the site's help pages.
1 - Preferred submission format: Any of the formats listed in the upload box.
Since I added .odt and moved to LibreOffice as the file converter, all the these formats are equal: HTML, .docx, .odt. .txt is of course supported, but its formatting options are limited. The sites formatting guide is extensive and include all this info. I didn't think it needed a help entry to point to it as it's a prominent link in the Authors/Editors page. I'll add pointers in the help pages to make this stuff simply searchable.
2 - Bookapy ads are in the 'Manage Stories' functions. Authors/Editors -> Manage Stories -> Click Story title -> Bookapy ad button on top.
3 - Also in the formatting guide.
ETA: After answering here, I went into the admin side to add the answers to the questions you asked and I found that I have already done that.
Click the Help link on Desktop view or the Question mark on mobile.
Type 'Preview Format' in the search field and #1 and #3 will be provided.
Type 'Bookapy ad' in the search field and #2 is provided.
.txt is of course supported, but its formatting options are limited.
I consider this a feature. 😎