If you don’t want to see the mall deleted from the server, use IMAP in K-9. That absolutely answers your real question, which is about keeping the messages on your server intact.
POP was never designed as a multi-client protocol. The option to leave messages on the server has always been a kludge.
If it is as important as you imply that the messages stay on the server, why expose yourself to the risk?
If you really want to see what happens in K-9 Mail using POP before you risk access to your historic mailbox, it will be more reliable to send some messages to a new test mailbox and examine the behavior firsthand.