You do know that a convertible SUV does exist? Now whether Hertz rents them, or you would want to drive one, is a different question. Also you are driving Hwy 1 in the wrong direction -- much more spectacular driving south, as you are on the ocean side of the road. Personally I would have done this trip as Grand Canyon, LV, Death Valley, Yosemite, SF and then driven to LA.

Alamo is on Century Blvd at LAX, a few blocks from most of the other car rental companies (long walk). But wouldn't you rent the 2nd car before dropping the SUV, transfer the luggage and just drive to the dropoff location -- so it really doesn't matter how close they are. The advantage of using LAX is that they have thousands of cars, so lots of choice (and the nicest cars seem to be at LAX).