Hello vux777 and thankyou very much. I think I found what you meant (Sandbox settings > Applications > Web browsers > Other) where there are options "Allow direct access to Opera bookmarks / to entire Opera profile folder". But, having selected these, changes to bookmarks are still not retained after deleting the Sandbox contents.
This is obviously a Sandboxie issue rather than an Opera one; but I suspect Sandboxie has not kept up with current Opera file locations. I am not technically proficient (at all!) so cannot tell from the underlying code for the Sandboxie configuration.
Hunting about in File Explorer, there seems to be a 'bookmarks' file (no file extension?) in AppData\Roaming\Opera\Opera Software\Opera Stable and 'bookmarks.adr' in AppData\Roaming\Opera\Opera (not sure if both are relevant). Maybe Sandboxie doesn't recognise these paths as ones to be allowed access?
Meanwhile I cannot make your 'V7 Bookmarks' stick either inside Sandboxie. After box deletion the extension just disappears completely...

This is helpful. Not having a simple bookmarks export function isn't. I use the browser inside Sandboxie to quarantine any nasties that might be encountered. Deleting the Sandbox unfortunately also deletes any new bookmarks added while browsing. At least the base set of bookmarks, which ironically were exported from a previous version of Opera, survives...