Today, you can buy print-at-home tickets from Interlaken Ost to Dusseldorf Hbf with train changes in Basel and Mannheim for 29.90 EUR. Travel time is 6 hours and 15 minutes and the quickest itinerary by train. Go to www.bahn.com.
There are a few direct trains from Mannheim to Zurich on Dec 1. The 6:33 is going for 29.90 EUR, the 10:46 costs 39.90 EUR. Takes 3 1/4 to 3 1/2 hours. Again, buy them at the Bahn website.
Duesseldorf to Mannheim trains are direct and very frequent, like every 30 minutes. Prices vary and take 1 3/4 to 2 hours.
Pretend that they are airline flights, then you don't have to worry about your schedule changing. You'll save a ton of money buying now, like 200 EUR.
If you want maximum flexibility, you could buy a 3-day German Rail Flex Pass. It is on special now for travel in your time window for 180 EUR. You will have to buy supplements for the sections in Switzerland. Then you can ride any train you want. It is more money than buy advance tickets, which should run you about 120 EUR if you buy now, but gives total flexibility.