For some reason Opera doesn't let you edit Forum posts, so posting the solution as a reply.
While in Full Screen Mode in Opera, if you can't see the address bar and/or Sidebar on Mac, move your mouse to the top edge of your screen, click "View", then click "Show Toolbar in Full Screen Mode" or use shortkey: [shift][command]F
Yes, I actually also figured it out by a sheer chance...
It looks like Opera's bookmark management needs some work:
-The Bookmarks menu opens the popup panel with very limited management abilities;
-However, the contextual Bookmark menu of the bookmarks bar opens the full Bookmarks page (the one that you mentioned)
-Then, if the user with a bit of luck discovers and understands Workspaces, they find out that there's a choice to always use the full page, in which case both Bookmarks menu and the contextual menu will open the full page.
In addition, the selection of multiple items is still implemented in a non-standard way: user has to first discover, then click a check mark at the far end of each bookmark. There's no way to select multiple items at once by selecting first and last item of the desired group while holding shift.
I'll submit a feature request for better bookmark management on an appropriate forum.