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1) If you have Output:EnergyManagementSystem in your *.idf with the first field (Actuator Availability Dictionary Reporting) set to Verbose you will find a file called eplusout.edd with your output files that contains all the available actuator unique names, types, and control types. Reason I bring that up is if you look for Outdoor Dry Bulb you will see that control type a couple of times, once for the Environment unique name and then again for every surface and sub-surface in your model. So to answer your question, if you choose the Environment unique name it would be the 'ambient air temperature' as you described above and if you choose the specific surface or sub-surface unique name it would be the 'external wall temperature'. The same applies for Outdoor Air Wetbulb and Outdoor Air Wind Speed as well. Additionally, you have both Interior Surface Convection Heat Transfer Coefficient and Exterior Surface Convection Heat Transfer Coefficient.

2) You can override those values for the environment, but I'm not aware of a way to do it for a specific surface. Maybe someone has some fancy way they'd be willing to share :) In addition to the ones listed above you have Environment control types of Outdoor Relative Humidity, Diffuse Solar, and Direct Solar.