Hello snowee812! First of all: whether you'll end up with plan A or plan B, both are beautiful trips! As for your questions:
1) It's hard to say which plan you should choose and it depends on what's most important for you: being in the alps or visiting the Chillon castle (which is considered to be a very pretty castle). As I said, both options seem great to me. My personal preference would be to spend as much time in the alps as possible, but that's a very personal decision. I'm not sure when your flight to Barcelona from Geneva on October 6 will leave. If it's (late) in the afternoon, you may even be able to combine your trip from Interlaken to Geneva with a visit to the Chillon castle, thus combining a 3 day stay in the Bernese Oberland with the castle visit. Depending on the train connections, the trip from Interlaken to Geneva takes 3h to 3h30, which means that there could be time left to get to Montreux on the way, if you leave really early. From Montreux, you can get on board of a ship that takes you to the castle in about half an hour. You can plan your trips using the timetable.
2) Yes, a Swiss rail pass is most probably worth your money, as you'll be travelling each day. Which pass is most economical depends on your exact travel plans though, which especially goes for the days in which you want to explore the Bernese Oberland. If you know what trips you want to make those days, you can start and make some calculations
to find out which option is best. Based on the plans you describe, I'd say that a Swiss Half Fare Card
may come in handy. The prices of the trips you're sure of (Strasbourg - Interlaken Ost, boat trip lake Thun, Interlaken Ost - Geneva Airport) amount to CHF 217.00 (per person and in 2nd class). This means that with a Swiss Half Fare Card, which costs CHF 99.00 and offers a 50% reduction, the total costs will be CHF 207.50. The Swiss Half Fare Card will also offer 50% discount on most trips you plan to make in the Bernese Oberland. As for other options: a Swiss Card
would in all be slightly more expensive. And as the Swiss Saver Pass
is concerned: I'm not sure if you'll be making enough long journeys to make this a cost effective option. Again, this depends on what you'll be doing. The Swiss Saver Pass offers ease of use though, as you don't have to buy separate tickets for trains, buses and boats. The Swiss Half Fare Card does require you to purchase discounted tickets for each trip.
3) The beautiful Swiss landscape can be explored perfectly by train and cable car. You don't have to hike to arrive at splendid locations. The boat trip on lake Thun will offer nice views (in clear weather of course). Lots of day trips which can be made from Interlaken are listed here
. For example, consider a ride up to Kleine Scheidegg via Lauterbrunnen/Wengen, maybe even up to the Jungfraujoch, and back again via Grindelwald. In Grindelwald, cable cars for Männlichen and First leave. So you can easily reach several stunning viewing points in only a few days. Cog trains leave from Wilderswil (to Schynige Platte) and Brienz (to the Brienzer Rothorn). For the ride back to Geneva, you may pick the route crossing Zweisimmen and Gstaad, which is part of the impressive Golden Pass route. The standard route, via Bern, is faster but less scenic. If you won't be combining the trip to Geneva with a visit to Chillon Castle, and if you have enough time on this day, the Golden Pass route will definitely be worth the extra travel time.
4) Interlaken is a convenient base for exploring the Bernese Oberland. It requires only a short train trip to get into the alps and it offers great connections to all directions. And as it's in between lake Thun and lake Brienz, boat cruises can be easily made. If you plan to concentrate on the Jungfrau region (the area stretching from Mürren to Grindelwald, south of Interlaken), towns such as Lauterbrunnen, Wengen and Grindelwald are very nice as well. They're situated less centrally, but are right in the heart of the mountains and offer a perfect base for mountain train and cable car trips. So again, you should try and decide for yourself what's most important.
I hope this helps a bit!