Hi Felicia -
Your questions are not unreasonable nor excessive. Keep them coming. ;-)
They are well thought out. And, I can see you are learning to use the tools for good traveling in Switzerland.
I'll try to give some quick answers, and either i or Arno /Annika can help out later, if I don't have time to finish just now.
In general -
I'd stay in Luzern on the last days instead of Zürich. Zürich is my favorite city in Switzerland, and can be reached in 45 minutes by trains from Luzern, but it does not reward the first time traveler the way that Luzern does. Luzern has excellent and frequent access to Zürich airport.
I'd also maybe consider a short visit to Bern, using the trams instead of walking if necessary.
Use the timetable to check travel times from Luzern and from Interlaken/wengen.
Finally, the choice of going to the Rhine Falls is a matter of personal preference.
Since i live in the USA and have visited Niagara Falls a few times, I did not find the Rhine Falls to be very special. However, your background can be very different from mine, and many people enjoy visiting them. I do recall that some approaches require a bit of walking, but it has been many years since we went there, and other approaches may be available.
1.- There is only one station in Wengen. The Bernerhof is less than 20o meters from the station. Slight uphill walk.
2.- I have not actually ever spent the money to go up to Jungfraujoch, even after 80 or 90 trips to Switzerland from my home in the USA. I prefer to visit the places that fewer tourists go to.
I am sure that there is only one train stop, and all the attractions are easily accessible. I believe that they are included in the price of the train ticket, except for the Snow Fun park. Arno can confirm.
3.- The trip to the Jungfrau involves sitting on a train. That is not very strenuous. ;-)
Once you are there, you walk around for short distances, all indoors. No special gear is needed, but if you step outside, it can be cold if it is not sunny. I would wear layers.
Do not buy your ticket until you are within about one day before travel time. The weather is critical, and if it becomes cloudy up there, you cannot see any of the scenery that you go up to see. Hotels/rail stations/ tourist offices all keep up to the minute information on conditions "up top."
4. If you wish to go up to Männilchen, the route from Wengen in the time table is "Wengen-Wengiboden" to Männlichen LWM" which stands for Lauterbrunnen- Wengen- Männlichen cablecar company.
However, simply "Wengen-Männlichen" will work.
Buy your ticket at the base station just before you ride up.
Once up there, if your companion's knees can manage, walk to Kleine Scheidegg. It an easy walk, slightly down hill, and takes an hour and one half to 2 hours. It is easy enough to be called the "Grannies' Walk."
Even without that walk it is a nice place to visit and have lunch.
5- Check the link from Kim that I gave you above.