Didn't test, but this might work:
Make a test standalone installation and close it. In its install folder, go in to the profile/data folder and delete the Bookmarks and BookmarksExtras files. Then, in the "Sessions" folder there, delete all files, and put copies (so you still have the originals just in case) of your old session files there.
If you're lucky, when you start that test Opera, you'll see your opened tabs. Then, you can right-click on a tab in the tab bar, goto "save", and choose "all tabs to a speed dial folder".
Then, goto the URL opera://bookmarks and use the drop-down at the bottom left to export the bookmarks to an HTML file.
Then, in your regular Opera, goto the URL opera://settings/importData, select "Bookmarks HTML file" in the drop-down, and import the HTML file.
Then, at the URL opera://bookmarks, right-click on that imported speed dial folder and choose "open all in tabs".