Update:
My sister talked to the tenant’s dad (his info was on the rent application). The dad confirmed that his son is in jail, that mom & dad have removed most of the tenant’s belongings, and that they have left the key under the mat.

(1) The fact that the key was returned is a good sign, I think. It means that the tenant has moved out and now it’s OK for me to go in, clean up, and change the locks. Right?

(2) BUT, it still worries me a little that we didn’t receive the keys from the tenant (who is in jail), but from his dad.

(3) The dad said that he couldn’t remove a couple of furniture pieces and an old TV. He said that I’m free to give them to charity. Still, this worries me b/c it means that the tenant is not 100% out of the house.

My sister will be talking to a real estate lawyer today. Thank you all for the feedback. I had a big laugh with that Nigerian scam post!