1. Yes... you need to export your book when finished, do they same with your preview ( a separate book with a minimum of 15 pages.) unless you are using a chapter of the book or allowing iTunesProducer to create your preview. Then using Publish should work out OK, because during the delivery process you need to select both the iBooks file of your book ( and preview if its a mini bbok you intended). Mant people try to upload their working file ..which is xxxxxx.iba. The .iba file opens your book in iBooks Author and is NOT used in the iBooks store.
2. Preview: you do not have to minimise the the file size by removing video. The idea of a preview is to allow the rerader and oppportunity to examine a sample of teh books contents which may help to decide a purchase of the full book. The Preview should be 15 pages minimum. If you have - say 5 chapters, you could leave in the video and delete everything except 3 pages in each chapter. The content is your choice- but needs to promote your book in a positive way.
Preview: Chapter - you indicate when asked and select the chapter you prefer to use.
Preview: Let iTunesProducer auto create - if you select this option - you do not have to upload any file.. iTunesProducer will auto select a suitable sample to use.
3. If you cretae a folder for each book you make, and move the books working file..ie xxxxxx.iba file type
if/when you export a book it will be placed in that folder also.
When you reach the section to select your book and preview, if you have created one, when you select, you need to navigate to the folder and select the iBooks file. I don't see how you can have two files with identical names... also if you create a preview, it needs to have a different name. I rename to PREVIEWmybook.ibook- but anything will be OK- as long as you understand your changed name.
This way your files should not create any problems.
4. If you need to ..come back if it occurs again. ( which it may if you need to update the book later ).
A useful item on your desk, is a notebook and pen! Use it to make notes on errors or create a checklist of everything needed in the delivery process and annotate their individual requirements.
In truth, you seem to have done everything correctly and apart from file names..it possible previous issues create anxiety when uploading a new book.
I always suggest having everything needed per book to be in its own folder:-
1. book.iba file ( plus any test version and backups ie mybookBU1.iba PLUS 1 preview book file (xxx.ibook) if using one.
2. 4 screen shots and 1 cover image at the correct size.
3. I use a plain txt file with my "description " I write it up and change as needed and simply copy and past when I start a deliver process.
I appreciate you may already do this, but I include for any newbies who may see this later.
Good luck with your books.....