Good plan or last minute changes??

Have you read this thread?
https://community.ricksteves.com/travel-forum/switzerland/covid-certificates-required-for-restaurants-museums-etc-as-of-monday-13-sep

You are worried about weather in Zermatt, but you have planned Viator trips to Jungfrau and Titlis on specific days with no idea what the weather will be. First, why Viator? Do you have free lodging in Zurich that you are returning there, only to take long bus rides back to Engelberg and Berner Oberland? You realize that you will be passing through these areas on the way back to Zurich. Second, if the weather is socked in at the Jungfrau on your chosen day, it will be a total waste. Ditto for Titlis.

Better might be to play the whole thing by ear when you arrive and can get a short term weather forecast. No reason to waste money on Viator, you can easily do it on your own. Buy 30-day Half Fare cards when you arrive for 120 CHF each and all train travel, mountain lifts, including the Jungfrau will be half price.

Do you have to stay in Zurich? Luzern is much nicer, more scenic, and more convenient. Its only a 1.25 hour train ride, from the rail station in the Zurich airport. Either go there first, or stay there last and go directly to the Zurich airport from there. The sights in Luzern dont need a guide - lake, view of the mountains, Lion Monument, wood bridge - they're all pretty self-explanatory. A trip up Titlis (or Pilatus) would take up most of a day in itself. So staying 2-3 nights would be better. You kind of have to judge the weather at the top, and go up when its not cloudy, which I think is more often true in the morning. Same with Jungfraujoch, which would take most of a day in itself, without actually seeing much of those other towns besides what you can see out the window. Those day trips sound grueling. The distances on the map are deceiving - it takes up a lot of time to transit in the mountains. And its hard to explain how easy it is to get around by train without a guide. All three mountain experiences depend on the weather, not just Zermatt.

With only 7 days, if it were me, I'd do Z Airport>Luzern>Lauterbrunnen Valley>Zermatt> Z airport, or the other way around.

That's a death March of a trip.
I agree with above, go straight to Lucerne from the Zurich airport.

From Lucerne - an hour boat ride to the cogwheel train that ascends Mt Rigi.
You also have access to Mt Pilatius, and the open air cable car to Mt Stanhorn. You need weather flexibility. Lucerne is a great city to explore if the mountain peaks are fogged in.
Maybe three nights- which is two days/ for Lucerne.

Skip Zermat- "one- trick pony" town , as RS likes to stay.

Berner Oberland- at least three nights.

Bern- train from Bern to Zurich airport- trains are fast and dependable. Bern is a great city for cloudy day activities . Bern is a quick train from the BO.

Have a great trip!

OK I think I'm totally revamping.

Is it reasonable to do this:

Head to Lucerne right from Zurich Airport
-3 Nights Lucerne
-3 Nights Grindelwald
-1 Night Zurich (night before plane back to USA - need to get COVID antigen test in airport before heading home)

That's reasonable! I might add that you could go to Grindelwald as soon as you land and spend 4 nights there, then 3 nights in Luzern and go straight from the airport from Luzern as it is only 1 hour and 15 minutes by direct train to the airport. I don't know when your flight would be, most flights back to the States leave early afternoon, leaving time to get your Covid test at the airport, or perhaps the night before in Luzern.

If you do want to experience a car-free village (like Zermatt), I would choose Wengen. Good jumping off place for the Jungfraujoch and you will know the weather situation when you wake up in the morning. Then you can get the first train up, before all those Viator people get there.

I think if I didn't have to worry about COVID testing I would be more ok with staying in Lucerne for that last night, but as it stands, I think going to Zurich the last night is better

I disagree with getting your test at the airport or in the city of Zürich the day of the flight. What if the results are delayed? What is one of you tests positive? You don't have any time left to check plan B which, if the results are slow may hose your flight, or if you or one of your friends is positive means you need to search out a PCR test and wait, often overnight? There goes that flight again.

You need to take your tests in the 3 days before flight. Easy to take out 30 minutes during the day before or even the day before that and visit a pharmacy or testing centre where you are. Almost always cheaper too.

I agree with the comments above, too. Wengen or Lauterbrunnen in your case makes much more sense than Grindelwald because you want to jump on a clear morning early to get up the Jungfraujoch. If you prefer to take a gondola part of the way up rather than the train, then the new gondola from Grindelwald may make sense, but understand that there have been very few reports here about how good it is because it is brand new, and that report spoke about the crowds on the train for the last bit up the mountain from the gondola - they had trouble getting on and had to wait. The train fills up when it leaves Lauterbrunnen and Wengen and nobody needs to get off at the station for the gondola. Another report noted that the train tickets are not valid on the gondola and v.v.

I completely agree that everything you have planned can be done cheaper and much more flexibly without Viator.

Regarding Covid testing-
You still have time to order the Abbott antigen self- test kits, comes with six kits. If you order before noon, you'll have the kits the next day. Reports here on the Forum report success with this form of testing . You'll need good wi-fi. You test yourself with an online proctor watching, takes 15 min for results, and you then receive results documentation via email. This is cdc approved.
Safe travels!

Have you considered renting a car? Agree with the idea of going to Wengen for a few days and then Lucerne before back to Zurich. I've visited to all 3. If it was me I'd spend 4 nights in Wengen, 2 in Lucerne, and your last night in Zurich if you have an early flight. With a car you have more flexibility and can make impulse stops as well as day trips (i.e. Bern or Interlaken if the weather is bad in the mountains).

Parking in those areas can be very expensive . Swiss trains run at least every hour to most destinations , and many areas have car-free zones. Stick to the trains!

I second taking trains. For the most part, trains run frequently and are on time. No one has to miss scenery while navigating or driving. Plus if you buy a Swiss pass there is no worry about taking a specific train and you will have the option to take a boat on many of the lakes without standing in line to purchase a ticket. And no fuss about finding a parking place which is often difficult and expensive to do in a city. I know from past experience. Whatever you decide, it is your trip. I hope you have a great time.

I think the Half Fare Card is the best option for this group. One trip to the Jungfraujoch and return more than enough to break even. The Swiss Travel Pass only gives a 25% discount on this trip, whether from Grindelwald or Wengen. HFC gives a full 50% discount.

Thinking ahead for my travels in June... trying to project how covid may affect sightseeing in Switzerland. Are you seeing required reservations in advance to limit the number of people at sights, etc? Obviously things are unpredictable and likely to change by June, just trying to keep up and plan.

@Kristin. Why not start a new thread on this topic. The OP gets notifications of answers to his/her questions, and no need to pester them with off topic answers.

Don't get a car for this trip and don't do guided tours. Switzerland is so easy to access on your own. Your arrival and departure schedule may determine which location to stay in first depending on train schedules to get there/back. On our last trip, we made the last (midnight-ish) train to Lucerne from the Zurich airport on arrival but had to stay in Zurich on our last night (traveling from Zermatt) to catch an early (6 am) flight the next day back to the US. Just look at your schedules and see what works best.

In terms of weather, it can be unpredictable at this time of year at any location, especially at elevation, so always have a plan B. Not sure if you have looked for accommodations that seem available on short notice (not as likely nowadays as it used be pre-internet) but keeping ultimate flexibility would be best if you are comfortable with the uncertainty. Otherwise, book your lodging and make the best of your two locations. You can always just eat and drink your way through Switzerland. :-) And you can hike and enjoy mountain topography on cloudy days even if you can't see the Matterhorn.