Just a thought...
Long ago I discovered that deleting the .mbp (data) file associated with the ebook file (Kindle\documents) resulted in the book opening to the beginning, as it would behave the first time the book is opened to read. However, if one has saved and synchronized notes and/or bookmarks, I am not certain what the outcome would be once a new synchronization occurred with a different Kindle.
I have not yet experimented with the additional removal of the .phl files (associated with collections) to determine if they too must be deleted to "reset" the book but I suspect not.
It would make sense that the deletion of at least the .mbp file (and perhaps the .phl file) would leave a "clean" file to share/sync with someone else on the account. My account is solo otherwise I would test my theory.