Update on river cruising in Europe:
I called Viking today and posed some of the questions being asked on this board.. I was told that the cruise of 10/5-10/12 went as scheduled itinerary wise (though no definite word on whether every port was reached by boat) and that all of the ships are at this moment SAILING from port to port. Since I'm not in Germany/Austria/Switzerland, I can't confirm this from first-hand knowledge...but that's what I was told.
I also asked about arrangements for early flights from Nuremberg back to the US, should water conditions prevent the ship from sailing into Nuremberg during your cruise. I was told that in the past guests have been put up overnight in Nuremberg hotel if they have had an early flight the next day, rather than being bussed back and forth. Again, I'm just the messenger.
I hope the water levels are perfect for your upcoming cruises and that the Christmas Markets are everything you've dreamed they'd be!