To answer your questions:
1) Yes it separates design from data. That is one of the powers of the tool. Write once, output into many different formats, including XHTML, ePub, PDF, MOBI, and a WXR file, which is a WordPress backup file.
2) Yes. You can export your data out in any of the above formats.
3) Yes. Theoretically, you could export your data as a WordPress backup file and import into any WordPress site.
4) Yes, again using WordPress. But Tony could also make all of the versions of the book available and people could pick their foramt. For example, he could export an XHTML package as a zip file and make that available for others to use using an HTML editor.
Hope that helps.