Great report! (well, other than calling PMC as PMU I took my first trip to Chile just about 3 years ago. I am taking my 8th this December. 4 of the trips = 10 days to 2 weeks in Chile each.

A few comments:
(1) You obviously didn't drive in Santiago - neither have I - even when I want a car to leave town, I'll go back to SCL to pick it up.
(2) I am really glad I didn't rent a car in Arica and do what you did, because I almost did, but there is not a single automatic rental in ARI. The altitude caused me a lot of grief and I would have had troubles driving.
(3) My first trip to PUQ I wish I had wandered more with my rental car - but it was my first time outside of Santiago and I was generally unfamiliar (and unnecessarily way over precautious)

I'm planning my December trip as we speak - the next time (going with someone who has never been) will be all back to familiar places. I can't wait!