Obviously any invasion of Switzerland is reckless, morally bankrupt, and could have huge diplomatic, financial, and political consequences, all on top of it happening during a cold war, but would it really have been all that difficult to pull off militarily?
The overwhelming majority of Switzerland's population and industry are situated in the plains and alpine foothills, not the mountains themselves. It seems to me that the invasion would be pulled off quickly and smoothly, excepting the military holdouts in the high alpines (and the destruction of critical tunnels and bridges).