If you are boarding in Regensburg on 9/3 we are on the same cruise. We are coming into Munich early as well but staying at the host hotel and arranged that with Viking with no concerns. If you were assigned a hotel by Viking in Munich and chose to move to that hotel on 9/1, I would think Viking would be obligated to include you guys in the trip to Oberammergau. For us, that is the morning of 9/2, followed by lunch in Oberammergau, the Passion Play, dinner at intermission, and transportation to the hotel which is about 20 miles outside Oberammergau
