Since there are only two ways to affect exposure. ISO and shutter speed on the Mavic, how can I possibly keep my shutter speed constant at 50 or 60 depending on my fps?  I am using ND16 and still have to raise shutter speed beyond 1/120 at times.  How can i get the best video when being forced to go beyond the 1/60?

If you're on ISO100 and getting fast shutterspeeds you need a bigger ND.  But don't use auto- that randomly uses a high iso and high shutterspeed for no reason.
If you're on iso10o and shutter is 1/120th with an ND16 then you need an ND32.

Darn it I just bought the Polar Pro 3 pack and it only goes up to 16!  Why don't they sell the Polar Pro three pack with the ND32 when on any given sunny day it seems that is what's needed. My question is what is the fastest shutter speed you can get away with shooting video? In order to lock in at 1/50 I guess go to an ND32 and up the ISO if necessary?

Good filters are always important to have for a better result.