Golden Pass from Luzern to Interlaken

It takes 30 minute to an hour or more to get off the plane, go through immigration, retrieve your bag and out to go catch the train. The trains to Luzern are so frequent that this is nothing to worry about. Though there is maybe one train an hour that goes direct to Luzern from the airport not requiring you to change trains. Forum expert Chris from Basel will probably chime here with more information.
But you can search the schedules here https://www.sbb.ch/en/home.html.

The Golden Pass line runs from Luzern stops at Interlaken on its way to Montreux (I took it in reverse order from Montreux to Interlaken the day I arrived and bought my Swiss Travel Pass at Geneva Airport). To be guaranteed a seat you have to reserve it in advance.
You can do that here :
http://www.goldenpassline.ch/en/
The train makes stops you can get off whenever you want including at Interlaken on your way to Gimmelwald.
Interlaken is popular hub.

Some people buy the pass before they go and some when they arrive. The only advantage some say of buying in advance is that if you think you are going to lose your pass while you are there then you can reprint it as it comes as a PDF in your email. I bought mine when I arrived and it came in a cardboard boarding pass like plastic sleeve and the map you need to see the area validity.

I absolutely would not buy it now as you have to choose the first day that you are going to be using it and the area of validity changes year to year. And you never know if your travel or flight plan is going to change between now and June 2020. If you buy it wait until next year a week before or if you absolutely know for sure that you are going to be there on the day that it is valid. The passes are good deal but expensive so you want to maximize using it.

I believe I read somewhere that Schilthorn won't be 100% covered in next year's Swiss pass. Only 50% off. But that's still TBC, there's no need to buy the pass in advance anyway. I wouldn't: what if you have to cancel the trip (I hope not!)?

Try to fill in some of the gaps.
First thing to remember is that all rail lines in Switzerland have at least one train per hour. Many have more (one train every 30 minutes). It is designed to be a "turn-up-and-go" service. None of that "booking seats" rubbish they have in other countries. Treat it like a lift (elevator) in your local high rise, turn up, wait for one going in the correct direction, get one, get off at the correct location - simple.
Second thing is it is a network of interconnecting routes. You cannot drive from Metropolis to Smallville going on the same road all the way without turning at any junctions, you start on Interstate 123. then interstate 456, then a state road and finally a county road. Same going from Zürich airport to Mürren, an intercity, then a regional express, then a local, . . .

I hope the above analogies help.

Q1) How long to get out of Zürich airport? - 30-60 minutes, but it doesn't matter as there is a train from "Zürich Flughafen" (the station at the airport) to Luzern every 30 minutes. You just get on the next one.

Q2) "To get on a Golden Pass train, will we need to transfer trains in Luzern?" - Yes (as above).
The "Golden Pass" is a marketing name for a route from Luzern to Montreux. It is not one train, you have to change trains, 3 or 4 are required to get from Luzern to Montreux:
Luzern - Interlaken - Spiez -Zweisimmen - Montreux.

Q3) "Do all Golden Pass Trains allow passengers to get off at Interlaken Ost?" - any train lets you get on and off at any intermediate station, that is why they stop there. And, you MUST change trains at Interlaken Ost. All trains terminate at Interlaken Ost, and you need to get on a different one.

Q4) "Can we buy a Swiss Travel Pass now, or do we need to wait?" - I very much doubt you can buy now for next June. Either buy online in May (SBB website), or at Zürich Flughafen station when you land. The price is the same both ways. No discount for advance purchase.

Q5) "I've heard that the lifts to Murren, Gimmelwald, and the Schilthorn, are going from included to discounted with the travel pass."
Here is a map of what is included this year: https://www.sbb.ch/content/dam/sbb/de/infotexte/uebersichtskarte-sts.pdf
You can also use that map for your planning, it shows all Swiss rail lines.
Up as far as Mürren and Gimmewald (which is really just an extension of Mürren) is public transport so the pass definately covers you. From Mürren up to Schilthorn is a tourist line, it was included this year, I have no idea whether it will be next year, presumably the marketing department knows, I don't.

Are you flying from the US to Zurich, or from another European city to Zurich? The reason we bought our pass in advance from the US is we did a cruise around the Med first, then flew from Barcelona to Zurich, and didn't want to have to rely on wifi connections while in Europe to buy the pass (which turned out to be a good decision, several places we encountered with free wifi required the ability to text a code for verification to be able to use it which did us no good without cell service), nor did we want to buy it in person at the train station with 2 children in tow (which felt like "just one more thing to do" with tired kids). I think we bought ours about 2 weeks before leaving.

Also regarding the Golden Pass, don't feel you have to take one of the "official" scenic trains. We took the "regular" train between Lucerne and Interlaken and the scenery was still stunning and easily observable out the windows. Never made a reservation in advance, never had a problem finding a seat, despite being in Switzerland in mid-July.

Enjoy Switzerland, it's a magnificent country. My kids won't stop talking about it over a year later! Rick's book was very, very helpful.

Great advice! Thanks for your help.