Reservations or not

Hello jimbo5! You're right: seat reservation isn't necessary for most trips in Switzerland. You can simply get on board of trains, buses, boats and cable cars whenever you want. Especially in September, when high season is over, this won't cause any problems. There are some exceptions: international trains may require seat reservations, which is indicated by an 'R' in a rectangular in the timetable. More information on this can be found in our rail FAQ. For special panoramic trains, seats have to be reserved as well. I don't know how you'll be travelling from Wengen to Lausanne. If it's by regular train, you don't have to reserve seats. If you opt for the panoramic Golden Pass train though, it would be wise to make seat reservations. This can be done online or a few days in advance at a larger railway station, for example in Interlaken.

Whether or not you can or even should make seat reservations, doesn't depend on the type of pass you use. With a Swiss Half Fare Card, it's possible to make seat reservations as well. The seat reservation fee is equal to all and it doesn't matter if you use a rail pass. The tickets for the rail trips themselves will be discounted by 50% with a Swiss Half Fare Card. By the way, a Swiss Half Fare Card indeed sounds like a good solution to me. You seem to have done the math already; maybe these tips will add some more information.