You have surely done your homework, and defined your question very well.
I personally always travel with a half fare card. I have the Swiss Residents' version which gives the same discounts....as far as I know, for all items.
Annika is correct in general.
You will mostly walk in the cities. The parts that tourists visit are generally walkable and not large. However, trams and buses are sometimes convenient, especially in hilly cities such as Montreux.
Frankly, I just don't worry about pocket change when I am traveling in the cities. I consider the normal cost of travel, shrug my shoulders, and pay it. However, if you are on a tight budget, you may have a different perspective. So, here is my best recollection of some of the costs.
In Zürich, a tram ticket good for a "short trip" (about 1/2 hour perhaps ) is about 2 .5 CHF with a half fare card. A regular tram ticket...good for maybe a couple of hours is about 3 CHf or 4 CHF. You can buy a variety of day cards, 24 hour cards (maybe 10 CHF with a HFC, good for exactly 24 hours) and a "9 Uhr...9 O'clock card" good after the rush hour starting at 9AM and good till maybe midnight, also for around 8-10 CHF. Matters are more confusing because trips outside the core of the city may be in a different "Zone" which is relevant for prices if you go outside the central core zone, which is usually named zone 110.)
Probably, the website of the local transport network in each city will give prices.
In Zürich, it is the ZVV (Zürich Verkehrs Verein) --
Lots of info, maps, etc.
Now it gets tricky-
You can look up each museum. My guess is 10 CHF perperson with a HFC. So, IF I AM CORRECT, two people, two museums is 40 CHF. But, it could be more or less, and some museums don't give free entrance with the Swiss Travel Pass, although many do so. And, an HFC does not always get a discount. I'd look up Chillon, in particular.
Assume that you use a tram in Montreux and Zürich....maybe 20 CHF total...
You are getting close to no substantial difference between the HFC and the STP.
Now- add in one or two spontaneous trips or events that you had not planned. TheSTP will cover the full fare on most public transport...you figured that part out correctly.
And, if the trams were free, would you use them to get around a bit in the cities to save your legs after a good deal of walking? Or, go up the hill to the Zoo in Zürich on the number 6 tram, maybe stopping to see the grave of James Joyce in the parklike cemetery up there? (If you are a Joyce fan, it is a special treat.)
www.google.com/search?q=grave+of+James+Joyce &tbm=isch&tbo=u&source =univ&sa=X&ved=0ahUKEw jVv6mE8ojKAhUKTCYKHRTq CCYQsAQIJA&biw=1149&bi h=627
www.literarytraveler.c om/articles/james_joyc e_switzerland/
www.fodors.com/world/e urope/switzerland/zuri ch/things-to-do/sights/reviews/jame s-joyces-grave-146413
I am not saying that you have to be a fan of James Joyce, or visit the Zoo. I'm just trying to give examples of things that you might wish to do on the spur of the moment, which are not in your plans today.
How about a lake boat ride?
My point is that having free tram/train/bus/boat travel can extend your distances and give you more time for SOME of the sites in the main cities.
That could make the STP the better deal.
But, you know, we are beginning to get into the range of estimating small cost differnces vs. the overall cost of the trip. At some point, a precise calculation becomes imprecise.....
Hope this helps...but, there is no simple answer once you get to the smaller costs. The long train journeys can be calculated precisely, and they are the major costs. It is the minor ones which are imprecise.