Thanks for the replies. It now turns out we are taking a car back to Munich instead so I can just get dropped off at my hotel and not worry about Friday night. Inner zone day pass for Saturday then just a one way back to the airport should be simple enough - but at least now I know the other options if plans change!

FWIW - did not have any issue using the ticket machines at MUC SBahn platform yesterday with my US-issued Amex card. It requested PIN and I just entered 0000 (was purchasing intercity tickets; not sure if thay matters).

I do have a working mobile here so will look to app as a backup.