Itinerary Advice - Germany/Austria/Switzerland?

Monday: Return to Amsterdam, fly to Frankfurt, train to Bingen [stay
in Bingen]

So you plan to go from Papenburg to Bingen by driving to Amsterdam, flying to Frankfurt and then taking a train to Bingen? Wouldn't it be a lot easier to just take the train from Papenburg to Bingen?

I'm afraid your trip's going to be somewhat of a blur. It's too many cities, many of which are not easy to get to. And it's too many miles.
Have you considered consulting a good travel agency with so many people?
We avoid the very expensive Switzerland--substituting Austrian Alps that are much easier to get to and travel in.
Your group might be worthy of a 14 person shuttle bus and an experienced driver to take you through a region.

Do as Badger suggests with the train to Bingen.

Tuesday shows no plans aside from the 1.5-hr. cruise. I'd advise a visit to Rheinfels Castle in St. Goar and a stop in Oberwesel or possibly Bacharach on the train ride back to Bingen. Nice walk around the old town wall in Oberwesel.

You are right - not enough time at each place as a rule. Too many stops, with some that "duplicate" other stops to some extent.

I wouldn't necessarily recommend both the Luzern and Interlaken stops. Just spend 3 nights in the Interlaken area. Saves you one night. The Berner Oberland area above Interlaken beats Luzern IMHO.

Also the Sunday business is way too packed and crazy. Don't see the point of all that at all. After 3 nights in Interlaken, just make the train trip to Salzburg and stay there for 3 nights. Skip the crowds and tourist mayhem around Neuschwanstein (the faux castle.)

From Salzburg you can daytrip into the nearby Alps for mountain lifts, incredible views, ice caves, salt mines, and castles.

Hohenwerfen Castle and Falconry show (in Werfen)
Koenigssee near Berchtesgaden
Eisriesenwelt
Jennerbahn lift near Berchtesgaden

If you are sold on seeing one of Ludwig II's palaces, there's one just an hour by train from Salzburg - Herrenchiemsee, on an island in the middle of Lake Chiemsee, near Prien:

https://www.ferienhaus-am-chiemsee.com/files/template/background.jpg

Crete: I love Crete. Spend all your time there. Lots of variety there, less partying, real places with real people.

Is this your first trip to Europe? Is so, do not plan on driving after an overseas flight (I won’t do it after many trips!). You don’t know how jet lag will affect you. And you would need three cars to fit everyone and luggage plus daybags.

Is this a multi-generational group of 12, or 12 friends in their twenties? Are you all early-birds? You’re going to need to get everyone up early and moving in the same direction. Trust me - this is easier said than done sometimes!

I would save Greece for another trip. It’s too much travel in too short of time.

Why would you fly from Amsterdam to Frankfurt when a train will get you there so much faster (4 hours) and cheaper, especially because you then want to go to Bingen? Buy your tickets 3 months ahead of time for the cheap fares.

You no longer have to buy train tickets 3 months in advance - you can buy 6 months in advance. So you can buy them now. With a group larger than 5, it looks like the trip to Bingen can be as low as only 23.90 each

For this compressed trip pick either Zugspitze or Jungfraujoch (I would pick the latter), you don’t need to see both if you’re trying to cram in a bunch of diversity this trip.

The train from Lucerne to Zugspitze is 7 hours. The drive from Zugspitze to Fussen is an hour. I’m not sure how you can squeeze the castles in there that same day. As others have said you need to pare back. With 14 days I’d keep it to 3-4 cities, max. Especially with a large group that may not be nimble to travel with or on the same desired timetable.