4: Regarding key frame distance - the DJI encoder uses a long-gap encoding technique which will generate a large number of B-frames (recording only incremental changes, not the full frame) before starting with a new I-Frame (full frame). The reason for this is mainly to allow encoding higher frame-rates (lots more data) to be fitted into the 60Mb/sec or 100Mb/sec respectively. The resulting "pulsation" can be really problematic.
I find that the flickering problem is a lot worse in mavic than in p4p. Do you know why that is? Also, why does it look like the flickering is particularly bad with trees?