2nd class is fine in all of these countries. The benefit of 1st class is more space and less people. You need to decide if it's worth the additional money spent. It's not needed, but nice to have.

If you do have travel insurance and a definite plan you might skip it. You pay for security. Same here: You need to decide if it's worth for you.

In general, trains in Germany and Switzerland don't require mandatory seat reservations (exceptionsare e.g. night trains and trains from München to Zürich via Bregenz). So you are free to decide which train you'll take up to the last minute before departure. On most routes trains do run very frequently. If you do miss a connection: Just hop on the next available train.

Even if you do miss the last possible connection at the end of a day, you should ask the staff for a regulation. You shouldn't need an additional travel day even then. But otherwise all you need to do is just to take the next available train.

Expect most connections in Switzerland to work. Expect more delays in Germany. 

Recommended: Use the national planner for detailed informations like real time timetable or track numbers. It's https://int.bahn.de/en for Germany and https://www.sbb.ch/en for Switzerland. Both do also provide smartphone apps.