Does anyone have further insight into why Hans actually leaves The Berghof after 7 years of living there? He can hear and see the war taking place, but Switzerland, I don't think, is not invaded. Why does he finally give up and leave?

I'm not sure that I would say Hans gave up. After Joachim's death, Han's seems to need something more, whereas, he had previously remained complacent in the sanitorium. His interest in Clavdia has waned, the people he socialized with in the sanitorium had changed, passed on, or left the institution, and his distance from real life seems to be closing in. Thus, when he leaves to take Joachim's place on the front lines, he has finally discovered something within himself... a purpose, even if it isn't a clear one.