Yes, since only a 100 waypoints can be loaded at a time it is possible to get through them in less than one battery when the terrain is rough.
It sounds like the warnings were correct. It is likely on the 4th and 5th flights there were less than 100 waypoints left to program, likely due to flatter terrain. Your flight logs aren't synced so we can't really tell more.
1. For that we are using the elements from the DJI UI and were attempting to keep it the same as the DJI app for consistency with that. We will look into it.
2. It is possible just depending on the spacing that one image was taken close to the edge of the survey area polygon and then it also tried to capture one as it left the polygon (which it always tries to do) but they were too close together and the second image errored. Again, we can't see your log so we can't tell more.