Dear respected Sam,
We went through your post carefully and we understand your scenario.
Here is one thing we'd like to first make it clear that no matter how many co-owners of a OneDrive file, the only thing matters is that the storage location of that file. It can only be saved in the original owner's OneDrive, which the URL of that file should look like
And once the storage location is deleted, the file will be gone. So after the original user left the tenant and his/her account will be deleted by the admin, and after 30days of Microsoft cloud temporary backup, all the data inside this account will be deleted permanently. And no one will access it anymore.
So, the thing you need to do is, contact the admin of your tenant (or if you are the admin), follow this official article to take out this important file from his/her OneDrive file to a new location (like download a copy and change a new owner) so that the related users can still collaborate on it.
See: Step 5 - Give another employee access to OneDrive and Outlook data - Microsoft 365 admin | Microsoft Learn
Let me know if you need any further support or I made any misunderstanding of your scenario. I appreciate your time and effort on checking these above and thank you for your patience and cooperation. I hope that you are keeping safe and well!
Sincerely,
Mia | Microsoft Community Moderator
