Hi wkiser, and welcome to my Swiss Alps.First, I'd say your plans are possible, and you'll see a lot of Switzerland. You clearly have done some homework.
It is always nice to see someone else besides myself from the USA who wants to write their dates backwards. Backwards as far as Europe is concerned, that is. I had to look at your title to decide if you were traveling in May or December. ;-). I've forced myself to use the European convention or spell out dates in this forum But, it is hard. ;-)
The entry page here may help you use the SBB timetable. It is a powerful tool, and a lot is buried in the menus.:
The directions on the splash page are a good guide to getting started.
If you were traveling after mid June, I'd ask why you don't spend more time enjoying the other wonders around the Lautebrunnen Valley. But , in mid May, the Valley itself is likely to be snow free...pretty much...and the higher trails are likely to be blocked by snow or closed for safety (snowslides). So, my first though- more time near the Jungfrau, less time in Italian Switzerland, is not quite so appropriate for that time of year, and personally I would chose Luzern as my base, as you did. That journey over the Gotthard pass is certainly a marvelous experience, and the boat ride starting at Flüelen goes through what many consider the most scenic part of Lake Lucerne (der Vierwaldstättersee).
Mid-May at Luzern's altitude will be Spring.
A trip to the Lauterbrunnen Valley is feasible from Luzern. (Do you speak or read German, by the way? I notice you use the German spelling, as I tend to do). 2 hours and 19 minutes inbound, once per hour. So, you'll have some time to explore a bit. The last train back - also once per hour - at 2132 takes 2 hrs 29 minutes and returns you at 1 minute past midnight.
However, from Interlaken Ost into the Valley to Lauterbrunnen and then up the mountains, they don't travel very fast. Narrow gauge and cograil equipped. Still you could go up to Wengen or Mürren and have a look from up there. Schilthorn is faster, easier and cheaper than Jungfraujoch. I recall that their Spring maintenance break is in April.
www.swissholidayco.com /Public/Assets/User/fi les/Map-of-Jungfrauregion1.jpg
The attached picture, taken after many many trips to get a clear day, shows why I like Wengen for views of the Valley itself. A couple others related to Wengen are for general interest.
<<"All travel in Switzerland will be by rail. Based on your website, I'm thinking the Swiss Half Fare Card would work best for our (wife & me) travel. I hope to book 1st Class, at least on the panoramic trains. Does the Half Fare Card seem the right way to go?">>
I leave that question for the experts and the spreadsheet that is provided on this forum, somewhere ....someone will link you to it. I never work that stuff out because a carry a 1 year half-fare card (used to be a 3-year card) , which I use for 2 or 3 trips per year. My first thought, however, is that with that much long distance travel in a short time, you should definitely check out the Swiss Travel Pass, as well as the Half fare card.
<<"Can the tickets on the panoramic trains/bus/boat be booked more than 90 days in advance? Same for seat reservations?">>
I notice that your question about reservations is narrowly related to the Panoramic trains. I think the answer is mixed, in that you may not need them for some of those trips, but will for others...I don't know about 90 days. Once again, the forum moderators know more than I do about that. However, If I were doing it, I would search the web site for those special trains by name. Actually, I'd start here:
www.myswissalps.com/ac tivities/scenictrips/t rain
Lots of information on the descriptive pages, and some of it may be useful in addition to reservation details I suspect from your phrasing that you know that for ordinary travel services in Switzerland...train, bus, boat...reservations are not needed and in some cases may not be possible.
That will be a great trip, especially if you really want to take those special train rides.
By the way, do you hike or want see any special museums?