Lake Geneva was pleasingly warm at start August and I imagine it will be just a little cooler in September (say 20C?). As for Montruex, not sure about the freedom to swim or the facilities for swimmers as my dip was in Geneva city itself. I can recommend Geneva's Bains de Paquis. Not at all expensive (2 Fr?), good facilities (nb the cafe) and a sign tells you the water temperature.

It was a lot of people walking on there, but I don't remember if there was cycle.

Yes most probably it will be autumn season and the weather will be cool. You will get to see some brilliant natural hues and floral display on the lake geneva. Have a lovely trip :)

Montreux has a microclimate, it's usually fine - but it's too early to be more precise.

Sorry, I don’t speak French... could you send me your question in English or Spanish please? And I’ll be glad to help you. Regards

It will be hot in August. There will be pretty flowers on the route .

It's only a few minutes in bus.

toward Chateu du Chillon