Hi Letty -
Welcome to the Forum. You are certainly lucky to have friends in Appenzell. One of the centers of old and traditional Swiss culture, and with some beautiful scenery.
Perhaps you will be there at the time of Appenzell Innerrhoden's Landsgemeinde, held on the last Sunday in April.
In my opinion, the train is by far the best way to see the scenery in April, especially since mountain driving could be treacherous. And, it allows everyone to look out the window and not have to concentrate on driving,
The routing of your trip sounds like it is well planned and would be an excellent rail journey.
I would have some concerns about April in Zermatt. I think that Arno and Annika (the forum moderators) know that area very well; perhaps they can comment. In other alpine regions that is sort of between seasons....the snow is not so good for skiing, but the trails are not open yet for hiking, and many facilities r shut down for vacations for staff, maintenance or lack of customers. Zermatt is at high enough altitude that winter may linger there , and the visit might be worthwhile. If you do go to Zermatt, some of the ideas below might be incorporated in your trip. If you decide not to, then the suggestions below might deserve careful study.
At lower altitudes, you will see the beginnings of Spring. April is often rainy with intermittent sunny days, but the weather is quite fickle in Spring.
I usually go to the countryside in May, but in 2014 schedule conflicts required that I move my trip to April. In mid-April 2014, there were one or two weeks of beautiful, sunny , mostly dry weather. Very unusual. I was able to capture a panoramic photograph on April 17th in the Emmental which shows the state of Spring. Plenty of snow on the Alps, the grass was only really bright green in the lowest regions, some fields were plowed and planted and the trees did not yet have leaves, but were in full flower. See attached, taken from the ridge above Sumiswald.
I'm not sure just how long the trip from Zürich to Milano via the Bernina route would take. The timetable will help you decide, but I think it would be a very long day to get to Milano that way. I think an overnight stop would be needed. Hope I'm wrong. The fastest route is probably via the Gotthard Pass.
A look a the map - here is a good one - could suggest alternatives.
You might consider going over the Gotthard Pass , to or from, Italy ...on the northbound trip, that would offer a magnificent rail journey to Luzern and thence Montreux. There are some high mountains within reach of Luzern, which might permit snowboarding. Might. Perhaps your husband would enjoy your service as a guide to Luzern. ;-)
Or, you might go north over the oldest trans-Alpine rail route through the Simplon Pass via Domodossola to Brig, then via the upper Lötschberg tunnel though Kandersteg on a very scenic route. (Of course, Zermatt is an easy trip from Brig.) That route over the Lötschberg would take you to Spiez...from which you could go to Montreux on the "Golden Pass" Route, or to Luzern via Interlaken and over the Brunig pass....also quite scenic. Thence to Bern, or even Montreux.
Whether any of those or part of them would work for you would depend a lot on transit time vs. stopover time, and could risk diluting your trip into a bunch of quick snapshots. The only way to tell is to work the time table. But,Switzerland's rail network gives you a huge number of choices.