I might have run into the Vortex Ring State now. I have been trying to push the limit of my tail motor ESC to the point of failure by forcing full throttle torque at the bottom of my dives.
2 incidences . The first dive starts at 1:00 mark. In either case, maximum throttle-pitch was applied at least 2 seconds before the touch down crash and the craft crashed with full throttle-pitch still in effect. The pull-up deceleration appeared weaker than with "regular" pull up moderate collective pitch.
The solution is to just go back to the more moderate pull-up that we instinctively develop.
I have a diagram my reasoning of the vortex , which is a stall vortex due to wind direction change with the dive itself, in attached picture.
I believe this happens to quadcopters too, just that no one tries to push the limit of the tail ESC because there is no tail motor there.
I have found my tail ESC that allows the the vortex ring fault to occur before the ESC fails.
John
John, just an idea.
You could consider doing the dive in a different way.
Do it fully upright, horizontal and apply negative collective.
You now have a fully 3D heli setup right.
You will not have any issues with 'retreating blade stall', not even for the tail rotor since it spins typically 4 * faster than main rotor.
However, you will likely run it to another issue, Vortex Ring State.
But, I was just thinking this should just be initially, and then you can push through it, descending much faster.
This style of dive will give same speed and sensation as 40-50 m/s but you can control the descending speed so it keeps under control.
This will allow you to use Attitude stabilization for simplicity but more interestingly, a much longer dive, like any distance you prefer, really.
Move the camera to a downward position, or have two cameras, or maybe a camera that can tilt.
This is the problem of Vortex Ring State and how to recover from it. But you don't want to recover 
But very little or no research has been done if you want to push it faster downward.
No one have ever had an interest doing that.
Fascinating John!