Ok- let's consider the OP's original question. The 3:2 image will, to use his words, give the fullest 20mp image. In fact it is the only available aspect mode that can produce a 20mp image. I don't think anyone is disputing that the 16:9 image seems to be a tiny bit wider across the frame. To speak for myself (although I would expect most who have even a casual interest in photography would agree) I am not discounting it I am saying it's of no consequence. To fly the additional altitude of distance required to make up for it is a trivial excersize.
If the 16:9 is a 'tiny bit wider' that means that the 3:2 is not using the full sensor doesn't it?
If it's not using the full sensor it cannot ever produce a genuine 20mp image
As we are getting 20mp DNG files something is clearly not right - see my post number 10 which shows what the Phantom captures 20mp and what it delivers - in this case approximately 15mp which means roughly 25% of the original recorded data has been discarded.
The final image has been distorted to make it look good but this means that things go out of position (rough illustration in post 30) so aligning panoramas is not as easy as it should be.
What is also contributing to the final image issues people are seeing it that we are presented with a 15mp image being enlarged to 20mp in camera.
I mentioned the difference in results from the two different formats a couple of times - if we gain a couple of cms at a few metres range that has to increase as we get to larger distances - as you say, the field of view determines that. If it were constant then it would be easy to compensate but as it appears to be decided 'in camera' it's a variable that a people didn't expect and some of us don't really want to have to deal with.