I wouldn’t worry, there is a 2 hourly direct service to Zurich from Munich that do not require reservations.

The thing is they say the train may not reach Munchen, so I was wondering of there were indeed case of such thing ahappening, the train stopping ag Schwandorf without offering an alternative

My daughter and I took this train on the 29th of July. We had seat reservations and that was good. There was one carriage missing so the persons who had reservations in that carriage were got reservations in the carriages that were on the train so the train was really full.

The train was about 15 minutes delayed enterring Germany even though there was no police control of documents as mentioned by DB. However the train wasn't allowed to continue to Munich after Schwandorf so in Schwandorf everyone had to leave the train. 

For everyone continuing to Munich there was a train waiting in Schwandorf so we got to Munich about 30 minutes late and there was no problem reaching the connection to Zurich. The new train from Schwandorf even had the missing carriage attached. 

People going to Frankfurt and other places not on the Munich route had to wait in Schwandorf for other connections.

It is as anna fra Sverige-now on trevlig resan tagluffare med dotter- tells: the worst that can happen is that the cars get on the next inland DE train to M: 1 hr later.

Incidentally today I also arrived in M-but will not stay long enough to catch Oktoberfest.

However, even though it is not mandatory-now in summer it is highly adviseable to RES the Giruna tilt trains M->Züri: heavy demand and overflow from the ultra cheap 9€ ticket here in DE means also very full trains. for 4,50 easy online on bahn.com-choose seat only.