Thank you all.
Yes I had thought about the "manual lock file" idea. We are three friends and we would impose the discipline to write a "namemmdd.lock" file (e.g. "bob0422.lock") on dropbox and delete it when we finish using it, and the batch file could be a great idea to add. The purpose of writing name/date is that if one of us "forgets" the lock there we can ring him/her up ask as to why and then carry on
Also the "dropbox as data vault out of calibre's jurisdiction with a "clean" and "to fix" directories" is a clever idea. If I get your idea well the "to fix" data would be "added" to a non-dropbox local librabry adn when clean exported "to disk" where the disk is the "clean" book with the mobi, opf etc files ? The only part I am not quite understanding is then how to use cailbre to load the clean books on my kindle (very new user hence I might be unaware of Calibre being able to use two different libraries).
Thanks to all and happy Easter.
Bob