I recommend that you use the DB Bahn search (http://reiseauskunft.bahn.de/bin/query.exe/en) to get an idea of the train routes for the places you want to go and the time it will take to get there. You can't buy train tickets except for Germany on it, but you can certainly see a map of the routes, the number of train changes and everything else relevant to making decisions on this.
If music is your primary reason for going to Switzerland and Austria, you also need to be sure that you are going to be there at a time when you can take advantage of that. It would be a serious bummer to get there and find that concerts were sold out or not happening or not something you want to attend.
If museums are important, be sure to visit the Bayeux Tapestry (http://www.tapestry-bayeux.com/index.php?id=322) when you are there. Most people concentrate on WWII and Normandy, but 1066 is just as important to history as WWII, and seeing the tapestry with an English language headset is a great way to learn about it.
Giverny is a little difficult to get to, but well worth the effort. It usually opens around the 1st of April. You can get to Vernon by train but will have to walk, take a taxi or take the shuttle bus to the Gardens. Make sure you can leave your luggage at the train station in Vernon if you are going through there on the way to Paris.
Now, totally personal opinion here. I would skip Switzerland and/or Austria in favor of more time in Florence. Much to my surprise, of all the places we have been, my husband found that a week in Florence and visiting its environs was not enough.
Whether you take trains or fly internal European airlines, you must pack light. I say this because sometimes people who go on cruises, even river cruises, take way more luggage than they can handle on trains or than will be allowed on the small European airlines. If you pack a standard 22" carry-on and one smaller carry-on only, and keep the weight down, your should be able to manage your luggage yourselves in all the ways you will need to do that when you get off the boat.
Maybe finally and especially since you are from southern CA, expect the weather to be wet, windy, cold and possibly snowy wherever you are in late March and early April. Consider any sunny and dry days to be gifts. Also expect to walk on sandy, wet, unpaved terrain when touring Normandy.