it is clearly necessary in fact to maintain a continuous descent to maintain a constant height above ground
Oh, terrific! Now we can get cracking on defining a descent, as there will clearly be some differing opinions on that one.
As for ground speed, define zenith as "above as defined by a plumb line" and I'll drink to the ground speed being the distance covered between being in zenith above point A and being in zenith above point B divided by the time taken to cover the same ground. Now, the question is... what distance?
(Above paragraph edited to avoid GS always being unity due to a brain fart in the denominator)
If you define instantaneous ground speed in the only sensible way, i e as the ground speed acquired when the distance between A and B approaches zero, we end up with it having to be the distance along the geoid. Otherwise, you'd end up with a difference between the distance A to B and the distance acquired when integrating the instantaneous ground speed over time.
And for those still suggesting that this complicating things unnecessarily, I suggest looking at the original post stating that this is for flight test purposes. What's good enough for plodding along from airfield to airfield is frequently no longer good enough in that scenario. Methinks this thread would have unfolded rather differently in the flight testing forum, without a lot of the "why on earth bother" posts.