If you delete the existing "Opera Stable" folder and copy your backup "Opera Stable" folder back or delete everything in the existing "Opera Stable" folder and copy everything in your backup "Opera Stable" folder to the now-empty existing "Opera Stable" folder (as in, you're not mixing new and old files), you should be good and your tab islands and the tabs in them should be restored. This will only work though if you're backing up and restoring an "Opera Stable" folder from the same user account with the same password on the same installation of Windows.
This will also only work if you make and restore the backup while Opera is completely closed. And, this will only work properly if there's zero corruption with the files in your backup. If you've been upgrading for a long time and your "Opera Stable" folder has been used by many versions of Opera (where some might have had bugs or crashes), your chances of things not working right will increase.
This will also only work right if the versions of files in the backup and still compatible with the Opera you use them with. Major changes to Chromium and or Opera might make a backup made a while back not fully compatible with the new version of Opera where things might break or be lost.
As for an "Opera Stable" folder that was using Sync, restoring should hopefully work given those caveats above. But, if anything major has changed with your Sync data or there are bugs with Sync, things could go wrong.
A for Syncing everything and starting with a new "Opera Stable" folder and then having Sync pull in all your data, things might not work right for tabs as tabs don't sync directly and tab syncing isn't tab-island-aware. It's also not workspace-aware. And, syncing settings has been known to overwrite or wipe out workspaces in some cases.
Besides those things, you should be good.
With that said, it's best to have exported backups of the things you can export just in case. For example, you can export your bookmarks to an HTML file at the URL opera://bookmarks using the drop-down at the bottom left. You can then import them at the URL opera://settings/importData by selecting the "Bookmarks HTML file" option in the drop-down.
You can export and import bookmarks at the URL opera://password-manager/settings.
For open tabs, you can right-click a tab, goto "save" and choose "all tabs to a speed dial folder", You can then repeat that for each workspace. Then, when you export your bookmarks, those will be exported too where when you import them, you can right-click on those folders and choose "open all in tabs" in the workspace you want. You can rename the folders before exporting to name them after the workspace you want. You can also use the opera://bookmarks page to move those folders to "other bookmarks" if you don't want them on your start page.
As for tab islands, you can ctrl + left-click each tab in a tab island, right-click one of those tabs, goto "save" and choose "tabs to a speed dial folder" to save just those tabs to a speed dial folder. Or, you can collapse an island, right-click the collapsed island, goto "save" and choose just to just save the island's tabs.
Saving your opened tabs into folders based on workspaces and tab islands that will and exporting your bookmarks will give you want way to restore your tabs and tab islands in workspaces if you ever have to. Not the ideal way, but it's something.