How to prefer networks from a specific country to avoid roaming charges?
Although I didn't understand your issue at all, I think this app (Roaming Control Trial
- INSADCO GmbH) would help you in some way.

It is trial, but if it functions on your tests, you can buy it - it will cost less than that small fortune...

With that, I think you won't create the priority list of network data connection, but you can simply reject all data from the Swiss carriers...