Well, I would love to be on this trip with you all! As you all know, I loved it so much I could easily do it over again.
I wonder if you will really need shorts? The first RS tour I went on was Heart of Italy which was the last 2 weeks of May 2013. It was really not uncomfortable in Cinque Terre to be in long pants as it was cool and rainy. I did wear crop pants in Rome though.
My shirt travel wardrobe consists of Lands End cotton modal tee shirts. For this time of year I would probably take 4 short sleeves and 2 long sleeves with 2 cardies and a Marmot Precip/waterproof jacket. Depending on what the temp was supposed to be in Amsterdam on arrival day, I would wear 1 short sleeve with one of the cardies, or one of the long sleeves if it's supposed to be chilly. With the 2 long sleeve shirts you can alternate them and probably get 3 days out of each before you need to wash which will put you in Austria. (Please know I probably would not wear them 3X before washing at home, but on the road?? yes, I would. Now does this make it so you would not want to travel with me? lolol) I would do a hand wash the night I arrive including the pants I traveled in. That gives you 3 nights for it to dry.
I am excited for you all!
Editing to add: OK, just did a quick look thru weather.com. Looking at the 10 day forecast for Amsterdam, Bacharach, Rome and Lauterbrunnen I would revise my suggestion to 3 short sleeves and 3 long sleeves. At the end of the 10 days it looks like temps will be in the mid to upper 50s in the colder areas. I could manage with short sleeves, a cardie and the jacket, but I could not have done that if I were coming straight from Florida. Been there, done that! If you have a polarfleece vest, I might consider putting that in for Switzerland. If you have a compression bag you can squish that down so it doesn't take much room in the bottom of your suitcase or in a side pocket. If you have lightweight glove liners I would probably throw them in as well.