Ground level photo

You should only include one ground reference image. You can include more but it is just a waste of points. 

If you took off at multiple locations for the 4 batteries, that is not a case that we currently support and the elevation products will likely turn out a bit funky.

Barometric Drift: Yes and no. We only take the picture at takeoff now. We used to take it at both and try to account for the drift but it was confusing too many people. We used to see drift of up to 1-2 meters during the course of a long flight. It was never THAT bad. The barometric reading will get reset for all subsequent flights so it isn't too bad over multi batteries. 

If you are really trying to make pro level stuff you will only want to fly under the most stable barometric conditions.

I was looking for clarification about including a photo taken while the UA is on the ground -- if my home point is on a building rooftop (~10m AGL), should my "ground" photo be taken on the home point, or on the actual surface of the ground? 

Will I have potential processing problems if I do all my battery changes during the mission on the rooftop if my ground photo was taken down on the ground? 

If you include an image that was used as the Ground Reference image that was taken 10m above the ground then all of your elevations will be 10 meters higher than they otherwise would have been.