If I was in your place, I'd try to track down the previous tenant. For instance, you could try to get address information from USPS (might work if the guy has a mail forwarding order) or you could try to find his address somewhere in government records.
If that doesn't work, I would return the items to sender.
Close family members can stay as long as the tenant wants
The tenant is entitled to "quiet enjoyment" of the property which includes living with their close relatives - spouse, de facto and children would all qualify; parents and siblings might as well. It doesn't matter if these people are children or adults.
You cannot contract out of this as you are not ...
No, it is not necessary to send certified mail
720.303(5) requires "a written request"; emails are a form of writing under law as are text messages, Facebook posts etc.
If a dispute arises, the onus is on the owner to prove on the balance of probabilities that "a written request" was made. 720.303(5)(a) provides that a receipt for "certified mail, return ...
Double check with your partner, in case they have a psudonom, middle name, relative, have changed they're name by deed poll or something.
Otherwise I think you should return to sender.
If it is legit. Then the sending company can try contact the correct person (say via phone).
If it is a scam, it doesn't help the scammers anything.
If it is spam, ...
As for a member of the Universal Postal Union, what repercussions will occur for country B?
Are they allowed to keep stealing stuff at the sender insurer's expense?
Well, country B isn't stealing stuff. Bad actors in the employe of country B are stealing stuff. This is a matter for law enforcement in country B. If country B is endemically corrupt ...