Avoiding tunnels due claustrophobic

I confirm that there are no major tunnels between Munich and Passau. There may be shorter ones taking a few seconds to cross, but even then, I do not think so either, the line is in a plain, then along the Danube.
As for the airport: I do not know for Munich, but I have never been to an airport where taking a lift was mandatory (outside of car park access). Somebody will probably confirm; I would not worry too much about that.

Edited to add that I forgot the obvious: the station at Munich airport is underground! But still roomy. Is that a problem?

Although there is one major tunnel you should be aware of, the 4 km S-Bahn tunnel under central Munich. To travel from the airport to the central station, make sure you use the S1. The S8 will route you through the tunnel.

the station at Munich airport is underground! But still roomy. Is that a problem?

If the OP goes from MUC to Passau, he could avoid the station by taking the bus to Freising and getting on the train there. The S-Bahn tunnel can be avoided by taking the Lufthansa bus from MUC to the main station or by transferring from the S-Bahn to the tram at the München Ost station. No mandatory lifts at the airport. No tunnels on the railway line from Munich to Passau apart from short road subways (10 metres or so, hardly noticeable).

I have never been to an airport where taking a lift was mandatory

For security reasons, every building needs to have a staircase as an alternative. In case of a fire, using an elevator would not be a smart thing to do. So, you may have an escalator (most airports and train stations do), but you will definitely have a staircase somewhere.

Same thing goes for hotels. They will definitely all have a staircase somewhere. Just make sure they don't stick you in a room on the 19th floor unless you want to consider that your daily workout.

but I have never been to an airport where taking a lift was mandatory (outside of car park access).

Terminal 1 at CDG.

The S-Bahn station at the Hauptbahnhof is underground. The route into the Hbf on S1 goes into the tunnel just after Hackerbrücke station. You could get off the S1 at Hackerbrücke and walk about a km by one of several serface routes along streets to the Hbf. However, I would concur with SLA's recommendation to take the bus to Freising Bhf and catch the train to Passau there. Freising is just 20 minutes from the airport by bus; from the airport to downtown Munich and back to Freising is an hour, maybe more.

For those not afraid of tunnels, since the building of the Neufahrn curve, there are now regional trains originating in the station at the Munich airport. However, they go via Freising and Landshut to Regensburg. You have to change in Freising or Landshut for Passau. From Freising there are direct trains to Passau.

Thank you everyone for sharing your knowledge of the local area. Invaluable 🙂