Hotels with points in Munich during Octoberfest: no chance, my friend. You can periodically check if anything opens up (it did for fellow FTers last year), but not really bank on it. Flights should be available without issues.

Hertz often has cars in Munich that need to be taken back to their origin countries. You can check the last minute status here: http://www.hertz-transfer.de/index.php?lang=GB

Otherwise I would just book a flight/train to Poland and start my rental there.