You've had a lot comments about which way to do the trip, at least partially in response to your question:
<<"Do you see any weather/open trail/service advantage in Zermatt or Oberland by reversing the trip and traveling:
Zurich to Lugano via train, then Bernina Express, Glacier Express, Zermatt to Oberland and Oberland to Zurich for flight out.">>
<<"Do you think the options with departure out of Zurich (with Oberland a little later in May) are better than my initial itinerary with departure out of Milan (Oberland earlier in May)?
Maybe more things open?">>
The answers you have been getting deal with the convenience and nature of the travel...long days, scenic routes, etc. but not specifically with trails, and facilities that are open. You can use the timetable and a map to work on the travel details:
The correct answer to your question about "more things open" is that there is no advantage from the point of view of the Jungfrau Region, to going later in your days in May, since you will be there in the first and second week. Until you get to somewhere around the last week, if anything, it will probably be about equally as bad in the Jungfrau region any of those days until you get into June. At the very end of May, the cableways that were temporarily shut down start up again, and you can get up to, for example, Männlichen, but the good trails aren't open until about June 15th.
You have advice from Swiss friends . It is good general advice. Most Swiss could not conceive of a reason to visit the Jungfrau Region in early May. Zermatt, maybe. There might still be skiing Zermatt...higher altitude.
For the the Jungfrau Region, you should try to make your plans to do certain specific things...such as go up to Jungfraujoch ( possible), or walk the valley between Lauterbrunnen and Stechelberg (likely possible.) Maybe the Trummelbach Falls will be open by then.......etc., etc.. There are places to stay, and, at worst, you could stay in Interlaken. Then you can test against transport schedules, and determine feasibility. Planning in general terms is not reasonable. Get specific. Especially since you have been there before, general advice saying that you can have a good time is not helpful. You will already have had your first experience being awed by the region. You'll want to do more than just be impressed. If you focus on details, then, you can set up a visit that makes sense, or decide to skip it..
But, if we talk about the Berner Oberland, the answer is different. The BO is much larger than and is not synonymous with the Jungfrau Region. There are lower altitude places in the Berner Oberland. Both lakes, towns and villages along them, for example.
This walk, in the Valais is nice at that time of year, and although only on the southern fringes of the BO, is easy to reach from Thun, for example:
A visit to the BO in general terms is within reason, and it might make a very very slight difference in weather if you go a few days later in May than you planned on you r first itinerary. But, not much. At that time of year, you could get good days or bad days and it's not possible to predict very far in advance. There won't be much predictable progression in the weather from the first week to the second week.
It will definitely be Spring...early Spring.
At that time of year, I like explore from Luzern. There are good hikes around the lake:
The lake boats have some kind of decent schedule, although not yet a full schedule. Luzern is quite as full of tourists.