A change of windspeed of 20 knots over 4 seconds, is what I would call instant. It would be foreseeable by the presence of at least light turbulence in the area you are flying. And no, if you descent without clearance and you are only calling pan-pan, you are breaking the rules. To deviate from any clearance you have to call mayday.
Bottom right hand corner of the HOLD page. It says BEST SPEED and gives a number.
Thanks for the pointer. Didn't pay much attention to that speed, it's the same speed as LSK1R (predicted target speed), if no manual entries are made.
So as you said yourself:
This prompted me to select holds in the cruise and compare the BEST SPEEDS for the NG, they too are significantly ( or slightly depending on what school you went to) slower than min drag.
So there you have it with your own conclusion above: Your margin to your "V drag too great for the thrust at hand to pull you out" is significant.
I actually wrote a longer reply to your practical holding observations, but deleted that again as it really doesn't have anything to do with the topic. I'll just leave it at saying: be careful about unfounded assumptions.