Additionally — Launch the admin client, turn on full-admin access, then try it.

zzzzzzzzzzz

sounds like you have a few problems here

Make sure on the ACL you are a Manager with all user roles assigned to your name.
If you can edit the ACL then you might already be a manager
Then make a local copy of the db copying only the design elements.
If you can’t open this new local copy in designer then the design elements might be hidden.
Goto the Admin client. Look up “Full Access Administration”,an assign make yourself a full access administrator then assign that tool to the folder the db is in. THe folder icon will change and you now have the highest possible access, even over-rides editor and reader access.
You can tell this be looking at the db properties, if some of the tabs are blankish or un-editable then it’s defo locked.
Search the whole domain and hardrives for a DEV copy of the db.
If you still have to luck them contact the person who left and get them to explain their actions.