Itinerary Tweaks and/or Suggestions.

That's a lot of big cities. Consider spending a few days in a small town here or there. Small towns are so much more relaxing and you're travel dollar can go further that way as well. Especially in the middle of summer when everywhere is so crowded.

Night trains aside, this order makes more sense ...
Amsterdam >> Cologne >> Munich >> Salzburg >> Vienna >> Prague >> (fly to) Paris >> London

If you are not committed to a flight into Amsterdam I would 'open jaw' through Prague ...
Prague >> Vienna >> Salzburg >> Munich >> Cologne >> Amsterdam >> Paris >> London

Nordheim makes a good point, especially valid for Holland. All of the Netherlands is well connected by train.
I would get out of expensive Amsterdam after 3 nights perhaps. Vienna is also a very expensive city.

I'm certainly no fan of train rides over 3 hrs.--especially night trains. And long train journeys are not very enjoyable in today's political climate.
Have you considered throwing budget air carriers into your itinerary?
You can travel cheap by taking EasyJet from Amsterdam to Prague--1 hr. flight.
Then you could take a train down to Vienna--about 3 hrs..
And the train ride from Vienna to Salzburg is easy.
Salzburg to Munich by is 2 hrs. or less by train.
You can fly Munich to Paris on Transavia-France Airlines--KLM/Air France's budget carrier. It's a much farther train trip than I'd want to take. (I've driven it 2x.)
And the Eurostar can get you back to London in 2 hrs. 15 minutes.
Cologne is a great day trip from Amsterdam by train.

Consider staying in Haarlem instead of Amsterdam. As remarked above, Amsterdam is expensive for what you get, while Haarlem is a most attractive city, 15 minutes from Amsterdam and some 30-45 minutes from Schiphol airport, all by very frequent train connections. For a change of hotel, consider Utrecht, a lively university city, or Zwolle, quiet and a good base for small cities and countryside around.
The suggestion of having Cologne a day-trip from the Netherlands involves a 2,5 hours train ride. One way.

Hi,

Nights trains that used to go Amsterdam, such as Berlin or Paris, have been dropped, but, luckily , not the Amsterdam-Munich connection. That's the entire route, start and terminus. It's direct and is a CNL night train, Germany's best. That route I would do on the CNL night train, which is also on my agenda the end of May, Düsseldorf to Munich. in light of the refugee crisis.

David (Brisbane), we have plane tickets at less the $800 each (it's really why we have them), so Amsterdam to London is set in stone. Munich and Cologne are not so much destinations as where night trains end. That they are nice places to spend a little time while before taking another couple hours of train is is a bonus. Were we to fly rather than train, we'd just skip them.

David (Florence), night trains are a matter of taste. We like them. They are a kind of adventure coupled with a night free. If one of us slept badly on them, we wouldn't do them. But I do see what you mean about the current political climate. I don't think heading from Amsterdam to Munich would be a problem, but Prague to Cologne might be. Prague to Paris by plane is a very good suggestion. I'll look into that.

Ton, I'll look into staying in Haarlem. We will use an apartment rather than a hotel because there are four of us and I like to cook from local markets. But if I can find a nice apartment there , Haarlem makes as much sense as Amsterdam, especially if it's cheaper. Thank you.

Nordheim, we are trying to stay in large cities with good small town day trips because there are four of us and apartments which tend to have three day minimums are cheaper for us. Prague and Amsterdam were top on our list because of all the small places reached easily and cheaply by train from there. But I'd love to hear suggestions for small affordable places in route.

Fred, let me know how your trip goes in May. That ought to be a pretty good indicator of how ours will go in June.

@ Jen...We may just cross paths.

I'm flying over towards the end of May, say the 24, 25, 26, or 27 and will be there the next 20 days, haven't decided on those particular details but the CNL night train is on the agenda, ie, from Düsseldorf to Munich, have a leisurely breakfast, then changing to Linz either crossing at Freilassing to Salzburg-Linz, or Munich to Passau to Linz. Depending on border control or border closure or even the absence of them, I should be at Linz Hbf ca noon, (11:30-12:30), which is just fine.