1) Here, I think, there are different possible ways the company could treat this in Oracle Payables, knowing the company deals with the vendor or employee on regular basis. But, what are those possible accounting treatments?

Also, what about if the payment was meant for a customer as a compensation payment following a customer complaint, but then the payment went to a wrong one-off customer who cashed the check and has since disappeared? Please, help!

2) Question 2 is more straightforward.
The company would ask its bank to stop the check with the given number. Mean while, the company will Initiate a Stop to the check. When the bank actually stops the check, the company will void the check in the Payables system.