How best to book Golden Express panoramic segments with regular segments?

I never use the fancy tourist only trains on that route - the hourly local Zentralbahn trains on the same route are modern and have panoramic windows all the way up from floor into the curved ceiling. There are tables, too, although mostly they are small ones. The train has no doors between carriages because it is all open, there are a few stairs on the train to get over the wheel wells as the train is low level and completely disabled friendly.

If you sit on the right-hand side, facing forward, leaving Luzern you will have the best view as you head up into the mountains and pass, and as you head sharply down towards Lake Brienz on your way to Meiringen. At Meiringen the train stops for a few minutes ( a vestige of when the locomotive had to change ends) and reverses direction to head towards Interlaken Ost. If you don't change sides of the table you will find yourself going backwards. By the way, the fancy train takes exactly the same route, including the changing ends at Meiringen). After you change sides of the table on leaving (or arriving so that you don't lose them when people join the train there, and you know what will happen ), you will again be on the better side for viewing the beautiful Brienzersee (Lake Brienz) and if there is enough water you will see Giessbach Falls again (you will have seen it coming down steeply from Brunig Pass) and the approach to Interlaken Ost.

You can only reserve seats on the Golden Express panorama train. On regular trains (which run every hour on all routes) you do not need to and usually cannot make reservations.

Whether you buy a full pass, a half price pass or individual tickets depends on how much you will be using transport in Mürren.
You need to buy 2 sets of tickets:
(1) from Zürich Flughafen to Mürren, via Luzern on Monday 14th August.
(2) From Mürren to Geneva, via Montreux on Wednesday 16th August.
These tickets will be valid on any train on that day. You can by them in advance, or at Zürich airport station when you arrive. There is no discount for advance purchase.
Then IF you want to go on the panoramic coaches you need to additionally get a reservation for those trains.

If you land at 08:30, you may make the 09:47 train, but you may not. The plane may be late, or you may have to wait to buy your tickets.
You only have 2 nights in Switzerland? As far as I can see you are flying from home to Zürich, landing 14th August, then on the 16th flying out from Geneva (back home?) This is a lightning visit.

Thanks for your help. We are stopping in Switzerland to break up the journey from the US to South Africa - in at 8:30 AM day 1 in Zurich and out at 19:30 day 3 from Geneva. I'm taking my 82 year-old parents on their dream trip which combines a safari with a Swiss Alps train trip. We are essentially doing the Golden Pass with a 2 night stopover en route in Wengen.

Based on the information that you've provided and my further reading, it sounds like a 3 day Swiss Travel Pass might make the most sense for us as it gives us flexibility and eliminates the need for us to buy tickets at the stations correct? Can I buy it now online and download it or print it at home and then simply board whichever trains meet our schedules? Then if we want to reserve seats on the panoramic train from Zweisimmen to Montreux I can also do that separately from home now for the specific train that fits our schedule?

Would you recommend the 1st class pass for us? I know there is not a huge difference in seating, but I want this to be as least stressful and most comfortable as possible for my parents so I'm willing to splurge a bit. It sounds as if the 1st class section of the train is not as crowded and thus involves less of a scramble for seats for the four of us. Is this accurate?

it sounds like a 3 day Swiss Travel Pass might make the most sense for us as it gives us flexibility and eliminates the need for us to buy tickets at the stations correct?

Yes, correct, once you have the pass all you need to do is board the trains. There are no ticket barriers in Switzerland.
The validity map for the Swiss pass is here: https://www.sbb.ch/content/dam/sbb/de/pdf/freizeit-ferien/ferien-kurztrips-schweiz/internationale-gaeste/sts-geltungsbereich_en.pdf
As you see it covers all trains up to Mürren (or Wengen). Above that you get a discount on the mountain lines.

Can I buy it now online and download it or print it at home and then simply board whichever trains meet our schedules?

Again correct, print-at-home is the simplest option. When the ticket inspector comes around (while the trian is moving), show the printout and your passport as ID.
You can gte it from SBB here: https://www.sbb.ch/en/leisure-holidays/travel-in-switzerland/international-guests/swiss-travel-pass.html

Then if we want to reserve seats on the panoramic train from Zweisimmen to Montreux I can also do that separately from home now for the specific train that fits our schedule?

Again correct!

Would you recommend the 1st class pass for us?

I wouldn't bother. You should have no problems finding a seat. If you can't find 4 empty seats together, you could ask someone nicely to swap seats.

If one of you has a phone which works in Switzerland, get the SBB app (details here: https://www.micro.sbb.ch/mobile/en/home.html ). This will give you all the train times in real time, including platform numbers.

Although second class seats are fine and all have a great panoramic view on the entire Golden Pass route from Luzern to Montreux, there are some advantages to first class that might be important to your 82-year-old parents.
1. Second class can get jam packed with tourists very competitively jostling for space for their suitcases, especially from the Zurich airport and from Luzern. Also from Interlaken to Spiez.
2. The first class reserved Panoramic-Express train cars on the Zweisimmon-to-Montreux portion of the Golden Pass route are special--their seats are like plush cushy lazy-boy recliners, and those train cars are decorated specially with modern or Victorian decor. The modern decor ones have the additional option to sit up front to see through the front windshield, while the driver sits in a bubble up above. (You would need to reserve and pay extra for those front-row seats.)

For your side trip to Wengen, the little train you get on at Interlaken Ost is a bit tricky. Half the train splits off to go into a different valley during the journey, so be sure the train car you choose to board says it's going to Lauterbrunnen, not to Grindelwald. There's no first class on that train. When you get to Lauterbrunnen, it's the end of that line, and you change to a littler train (a cog rail) that takes you up the mountain to Wengen (again, no first class).