"1) What was going on in Idaho in the early 20th Century that could have drawn a teamster/miner/adventurous spirit such as my grandfather and one of his sons out there, and why again in the late 1920s/1930s for the other son?"
Gold & Silver mining was the big draw at the beginning of the century. Idaho was growing quickly, with a lot of opportunity for enterprising individuals. My family moved to Blaine county permanently at that time, although they had also been here in the early 1880's. My grandfather had worked in the gold fields East of Boise in the 1890's, and moved to Blaine county to work in the Silver mines here.