Randa to Zermatt and Swiss rail pass

1. I am unaware of any "promo codes" for a Swiss Travel Pass. I don't think they operate that way. The prices are always the same, except when they raise them.
2. No prebooking required or available.
3. Randa is next to Taesch, which is the car park for Zermatt. Of course you can use your pass for that. A more interesting question is where are you coming from and if a Swiss Travel Pass is the most cost effective product for what you want to do.

Coming from the UK and travelling to Zurich, Interlaken, Bern, Luzern, Zermatt and then on to Milan.

Whilst in Zurich we are going to the Rheinfalls and whilst in Interlaken, we will be going to Lauterbrunnen, Shilthorn and Jungenfrau.

Sorry for poor English.

They occasionally have seasonal pass specials and Rail Europe occasionally offers dollars-off coupons, but none that I'm aware of at this time and never announced in advance.

Then a Swiss Travel Pass might work for you. You will still need to pay 50% cost for the ride from Muerren to Schilthorn and back, and 75% cost for the ride from Wengen to Jungfraujoch and back. If you take the Gornergratbahn while in Zermatt, that is covered by 50% as well.
It will also include local transport in Zurich and Bern. And most museums.
Assuming you will go from Zermatt to Milan, the pass covers you from Zermatt to Domodossola, Italy. You only need to buy a ticket for the train beyond Domodossola.

When going to Milan from Zermatt, we are going to take the train from Zermatt to Zurich and stay overnight there, pick up an early morning train to Chur where we will take the Bernina train to Lugano and then a train from Lugano to Milan.

Les, On your leg from Zermatt to Milan via the Bernina Pass route, why are you going all the way north to Zürich, just to stay overnight and head south again to Chur?
Zürich is usually not high on the list of places to visit in Switzerland. Instead, from Zermatt, after Visp (where all trains from Zermatt go to), continue on the train east all the way to Chur (you may need to change), and stay overnight there. Look at the map here: https://www.swisspasses.com/railpass/overviewmap_en.pdf
This is the route of the Glacier Express and very spectacular.

You mention the "Bernina train". This is not just one train, all rail routes in Switzerland, including Zermatt - Chur and the Bernina route have one train per hour, you can pick any train that is convenient. The Bernina route does not go to Lugano, it goes to Tirano in Italy (look at map). From there you can catch an Italian train to Milan.

You say your are coming from the UK, but how? plane or train?
As other said, you do not book seats in advance in Switzerland, whether you have a normal ticket or a pass, except for the Glacier Express and Bernina Express, and that is just for these trains, not "normal" trains on the same route.
The map I mentioned above is the official map that shows where your Swiss pass is valid.

And of course, you have the option of taking the actual Glacier Express to get from Zermatt to Chur, if you go when it is operating (until October 23, then starting Dec 11). There is a reservation fee for that one, or take individual regional trains along the same route. Take the Bernina Express to Tirano Italy and just take the regional train to Milan from there. Unless there is some special reason for going to Lugano.

And as with all passes, run the numbers for the trains you will actually use versus the cost of the pass -- and don't forget that you can buy a one-month Half Fare Card for about US$120 per person and then pay half price on point-to-point trains including the lifts to Jungfrau and Schilthorn.