>> What is the best time to travel through Switzerland if your thermostat explodes if the temperature exceeds around 25C?
I can understand your preference for travelling in cooler weather, and I agree that September is a great month to travel in Switzerland - the weather is pleasant, the crowds have thinned out, and all the mountain transport is still operating.
The Wengen "regulars" say that the first half of September is best for visiting Wengen. You could start off in the higher altitudes, and move to the lower altitudes later in the month (if your trip lasted that long).
However, given you have family on the German border with Switzerland, and will perhaps take the opportunity for several trips over the coming years, you might want to think about timing some of your trips to include the following:
---A winter experience with lots of snow.
To be sure of good snow cover, I would go in February, but not during the school winter-sport break. To give you an idea of the timing, for 2017 the winter-sport school holiday dates were as follows:
St Moritz 6-17 Feb
Grindelwald, Lauterbrunnen etc 25 Feb to 5 March
Zermatt 4-12 March
You can check school holidays (Schulferien) here:
www.feiertagskalender. ch/ferien.php?jahr=2017&geo=3056&kla sse=5&hl=de&hidepast=1
This is when the cows come down in a procession from the high alpine pastures before the winter snows set in: called the Alpabzug in German, or désalpe in French. These processions happen all over the alpine regions between about mid-September and mid-October, but not all of them advertise firm dates far enough in advance to allow you to incorporate them into your holiday plans. One that does is that in Seewis im Prättigau (Graubünden), to which I went last year:
---Swiss National Day on 1 August
I have yet to spend Swiss National Day in Switzerland, but there are major celebrations all over the country incorporating cultural activities (such as the wearing of traditional costumes, yodeling, alphorn playing and flag throwing). Some places also have fireworks. Chances are it will be hot though!
---Christmas markets; and Christmas itself in an alpine village
The Christmas markets are magical, and there is already quite a lot of information about them on MySwissAlps. Many towns also have beautiful Christmas lights and decorations. As for Christmas itself, snow will not be guaranteed unfortunately, but if you pick an alpine-village hotel that does a special Christmas dinner, it is a lovely experience. And you might be lucky with the snow!
---Chalandamarz in the villages of Graubünden on 1 March
www.graubuenden.ch/en/ discover-our-regions/stories/chalan damarz
---Assorted cultural and other events