Would these boots be appropriate for summer backpacking, will they breathe? Also, is the waterproof keen dry part built into the boot, or is it a separate bootie? Thanks.

Should be appropriate for summer hiking, as Keen Dry does have breathing/vapor releasing capabilities. Keen Dry is a membrane built into the boot. Theses are also equipped with a moisture wicking lining. If you're doing quicker hiking with a lighter load check out the Keen Voyageur Mid hiking shoe, it has more mesh on the sides then the Oregon PCT.

The Keen Dry insert is between the outer leather layer and the inner sock layer, so it is not a seperate bootie. Yes these would be appropriate for summer backpacking, I would suggest wearing a lightweight sock though.