I haven’t been on the RS tour but we have been to Switzerland several times (5 hiking trips, 2 weeks each). I don’t think you would make much use of the 15-day pass. Have you looked at what is included in the tour price? It looks like several boat trips and a trip op the Schilthorn.
So that leaves your free time on the tour—and there isn’t much of that. You do have a full day free in Lugano, and the tour description suggests you ride the funicular up Monte San Salvador. The current cost of that ride is 30 CHF round-trip, or 23 CHF one-way. If you have a pass, it would reduce that by 50%. So you could save, at most, 15 francs. What else would you use it for during the tour? I don’t see that you have time to hop on a train and go to another village that is not on the tour.
The 15-day pass, with the current discount, is 385 francs. This is the same as the combined cost of the 3-day and 4-day passes you are considering for before and after the tour (174 and 211 CHF respectively). But the 15-day pass will only cover your pre-tour days, not both your pre-and post-tour days. So you are better off spending your 385 CHF on the two passes for pre-trip and post-trip travel.
But depending on the number of lifts and trains you will use during your time at Zermatt (1 full day) and Wengen (2 full day), you could save by buying a 4-day Flex Pass for 243 Fr instead of the 2 consecutive-day passes.. You can use the Flex Pass to cover the full fare from Zurich Airport to Zermatt on Day 1 (regular fare 106 CHF), then Zermatt to Luzern on Day 3 (fare 106 CHF), then after the trip from Bern to Wengen (this is only 39 CHF so I have an alternative suggestion below) and finally Wengen to Zurich at the end (fare for this is 76 to 880 CHF depending on the specific trains used).
The Flex Pass will not give you a discount on any of the lifts you ride while at Zermatt and Wengen, unless you ride them on your travel day (which is an option if you are just sightseeing). But a consecutive-day Swiss Pass will only cover 50% of the lift cost, so you would need to look up each one to decide if it is worth the extra cost for a consecutive day pass for these excursions. Wengen to Maennlichen, for example, is 52 CHF for the round trip, whether you use the cablecar up to Wengen or the train between Kleine Scheidegg and Wengen. The pass would bring this down to 26 CHF. Or you would decide to walk down to Wengen from Kleine Scheidegg and save the 26 CHF that way. That is what we do.
The most expensive lift or train you might consider, I am thinking, would be the Gornergrat train at Zermatt. You might want to take this up to the top on your one full day at Zermatt (great view over the glacier), and then walk back down—-it is a beautiful hike with the Matterhorn in your face most of the way (if it is clear). The full price of a Gornergrat train ride is 88 CHF round-trip, or 44 CHF one way if you ride up and walk back. If it is a good day when you are there and want to do this, you could use a day on your Flex Pass to cover 50%, and then you are free to ride both ways, or walk partway down and ride the rest. You could board the train at any of several stopping places on the way down, showing your activated pass for that day. (This is what we did).
If you choose to use a day on your Flex pass for this—-and it is only worth it if the day is nice——then you would pay the 39 CHF for the train from Bern to Wengen after the tour.
You mentioned you want “ease of hop on/hop off travel” but remember that if you are riding a lift or mountain train, you only get a discount with your pass, not a free ride, so you still have to buy a ticket at the window or kiosk before you can board. So It’s not really hop on/hop off ease like it is for trains between towns.