Welcome,
Amsterdam in 4 nights = youre Good to Go (GTG). depending on what you like to do/see, it maybe too much time. you dont mention any side trips here, but others you do.
Brussels in 4 nights. . But i dont know about side trips. I spent my 3 days in Brussels only and didnt feel rushed in what went to see/do. I believe Bruge is about a 15 minute train ride. I was looking into going there as a whim, but changed my mind. Cost wasnt even a factor.
Paris in 7 nights. as far as the sight seeing in Paris. I spent 4 full days there and could have spent more but thats for another time. I went to Normandy for 3 full days too. In my opinion, going to Mont Saint Michel (MSM) is going to be a full day for you anyway you do it. I took the train up to Bayeux and stayed there. I then took the MSM shuttle from Bayeux. I think it was about 2 hour ride from Bayeux to MSM. The train i took to Bayeux from Paris took about 2 hours one way too. So if you take that route, its about 4 hours just to get there. Im not sure about driving so will let others that love to sit behind the wheel give their experience.
If you go to Disneyland, it will probably eat up a full day or more. just something to think about. Versailles will take a day also.
London for 7 nights. I stayed in London for 4 full days and saw just about all i planned to see, with a few exceptions. I didnt take any side trips either.
Q6.
as far a passes go, you will have to do the math to determine if its worth it to you. I bought the museum pass in Amsterdam and Paris. London, i didnt since most museums are free. I did buy an oyster card to use the tube in london and i still have it and use it when i return. Amsterdam has trams/buses/trains and you can buy a ticket for one trip or 1 or 3 day pass. Its up to you. I did buy the Brussels card since it worked for me $$ wise.
im like you in that i love the trains over in europe and will use it most of the times when possible.
there are several hotels/places to stay in the Normandy area or if you choose, there are places to stay on MSM. that would be the cats meow.
Q7.
Rick Steves books has some itineraries in his book. some of the others do too.
Everyone is an individual and what works for one wont or may not work for others. Its up to you to figure out what you like to do or see. the people on here can help with the details, but you have to provide more info.
I like to plan for about 6~8 hours/day at most of sight seeing and the rest of the time is reflection, people watching and extra time at an attraction if i choose to. Its not the best way or the right way, its how i do it and want to do my travels. Some may feel i waste valuable time, but it works for me and i dont feel rushed at the end of my day or trip.
If this is your first trip, consider it a learning experience. Once you get this under your belt, you will have a better idea on your next trip.
happy trails.