To return to this issue: we have investigated this issue to the fullest extent that we can, and we have come to the conclusion that this is a bug in the Google Drive app. On iOS, in order to allow other apps to access files, app developers need to provide a file access extension that manages the secure transfer of files between different processes. The extension provided by the Google Drive app only supports a very limited set of file types, and Dorico projects are unfortunately not among them.
We are trying to find a contact at Google to report this problem to, but unfortunately we have as yet been unable to find one. In the meantime, there’s nothing more we can do to make it possible to open Dorico projects via Google Drive in Dorico for iPad.