What benefits did the conservative military lifestyle led by the jamisons confer upon the young kay jamison? with what disadvantages did that same culture, with its stiff-upper-lip creed, afflict her in her battle with mental illness?

One of the most obvious benefits would be the disciplined attitudes that are brought by the military lifestyle, which makes young Kay Jamison excel at several aspects such as time-management/task efficiency.

The disadvantage would be that in that lifestyle, it is very likely that a child wouldn't be able to freely express his/her emotion (because it often ended up in backlash or punishment). This definitely would become a burden for Kay in the battle with mental illness.