>> Switzerland in early May ... landing in Geneva and leaving from Zurich ... Exploring Geneva on foot.
If you would also like to use public transport in Geneva for free, please read this:
>> heading to Montreux via train in the morning. - Exploring Chillon Castle on the way and Montreux through the day. - Now the initial plan was to go to Zermatt via Visp by the end of day 2 but I also want to look at doing part of the Golden Pass (as I believe its free with the Swiss Travel Pass) ... is there a way to do parts of the Golden Pass from Montreux but also get to Zermatt by the end of the day?
The key to finding out what you can fit into one day is to use the Swiss Timetable to plan your trips and see how long they will take. Daylight in early May will be from around 7:00 until 20:00.
(Use the one on the right with the red SBB logo.)
Between April and September, Chillon Castle is open from 9:00 until 19:00 (last entry at 18:00).
As you are short on time, I would skip exploring Montreux in favour of a trip on the Golden Pass route, which is very scenic and is indeed free with the Swiss Travel Pass. The most scenic stretch is between Montreux and Zweisimmen. You will have to return to Montreux the same way. If you don't have enough time to do that, consider going as far as Montbovon or Les Avants and then turning back.
>> The main reason for seeing Zermatt was because of the Matterhorn but I believe the cable cars will be in maintenance on those days (May 6-7 2018). Can you confirm this if possible.
You can't ascend the Matterhorn, but you can view it from various viewpoints.
The viewpoint with the "classic profile" view of the Matterhorn is the Gornergrat, reached by cogwheel train. This will be operating in May.
The cableway from Zermatt to the Klein Matterhorn/Glacier Paradise viewing point is closed for maintenance between 30 April and 11 May 2018.
>> Jungfraujoch ... am I right in saying that even with the Swiss Travel Pass, we have to pay another 105 pounds p/p to get from Interlaken to Jungfraujoch? is that correct?
With a Swiss Travel Pass, you get free travel as far as Grindelwald or Wengen. Between Grindelwald or Wengen and the Jungfraujoch, you get 25% discount. For accurate information about the fares, you can read the (full-fare) pricelist here:
www.jungfrau.ch/filead min/Prospekte_und_Bros chueren/Jungfraubahnen _Fahrpreise_Preisliste .pdf
Please note that the price varies according to the season of the year, which is intended to help distribute the demand over the course of the whole year. May falls into Season 1, a shoulder season. You would get 25% discount on these fares.
There is more information about the Jungfraujoch here:
You will see that there is a circular route up to and back from the Jungfraujoch. You can go up via Grindelwald and down via Wengen and Lauterbrunnen (or vice versa).
I have attached a diagram that might help you visualise both routes. Most people go up one way and down the other, as each route has very different scenery - all of which is dramatically beautiful.
If going up to the Jungfraujoch is too expensive, consider a round trip Interlaken Ost (or Zweilütschinen) > Grindelwald > Kleine Scheidegg > Wengen > Lauterbrunnen > Zweilütschinen or Interlaken Ost. That will be a lot cheaper and there is plenty of jaw-dropping scenery on that route.
The Schilthorn, which is reached by cable car from Mürren, is free with the Swiss Travel Pass.