Just to close this loop – I have managed to fix things.
Still not clear on what might have gone wrong, since I cannot think I did anything out of the ordinary on this one project, compared to the others, but so be it.
For anyone in future that might stumble across this thread, some points:
1.) I did not fully appreciate the fact that deleting a project on my iOS device, appears to delete the .Scriv project from Dropbox as well… This is obviously quite significant.
I noted the ominous warning that flashed up when I did it, but assumed that this was telling me that there would be no way to undo the delete on my iOS device - whereas I gather this pop-up was trying to tell me the entire project will be deleted, everywhere…
Maybe I alone failed to appreciate it, possibly as a result of how some of my other Apps work - I assumed it would ONLY be killing the iOS instance of the project – when it in fact, wipes it all, everywhere.
Something to keep in mind!
2.) Backup, backup, backup.
I have my Scriv projects set to back-up to a folder on my MBP, which is in turn backed-up to 3 different externals using a combination of TM and CCC.
I also have Scriv’s preferences set to back-up on opening, AND on closing any projects. And I regularly hit CMD+S by force of habit. Overkill maybe, until you need it!
When I wiped the project on my iDevices, and noted that the .Scriv file was then also wiped over in Dropbox, what could have been a major issue, was resolved a few moments later.
As in - open up the local backup folder, find the latest zip file, unzip to desktop, drag .Scriv project back into Dropbox, wait for sync, back in business.
3.) So, to be clear, after not managing to get either iPhone or iPad to properly open the project, I basically nuked it, and then dropped a fresh version into Dropbox. After letting the necessary sync UP take place, I simply re-synced on my devices, and all working again.
Joy. 