"Turning 'downwind' in a constant airmass with a fixed angle of bank is going to have the same performance impact as turning with the same angle of bank in a non-moving airmass"
Please consider this as I have witnessed this many times. Ie holding in very strong winds.
I reduced angle of bank to prevent excessive IAS loss.
Ground speed heading west is IAS minus wind. ( the Perth Westerly, sea breeze can be quite strong in high temperature days)
Ground speed heading east is IAS plus wind.
Inertia must be increased quickly therefore IAS will drop until ground speed equals IAS plus wind.
Sometimes this requires over a 50 knot increase in a short time.
Mmm....
This sounds suspiciously like you're heading into the oh-so-classic argument I've seen posted on various flying forums over the decades that goes something like "Well if you want to avoid a stall always make sure you turn into the wind, not away from it..."
This thinking that there's a difference in aircraft performance turning upwind versus downwind and that the aircraft's momentum needs to be taken into account when considering turning upwind or downwind is one of those fallacies that just keeps popping up again and again and I've seen some pilots swear black and blue that it's definitely something to be mindful of and keep in your tool bag of tricks.
As the first poster said "Turning 'downwind' in a constant airmass with a fixed angle of bank is going to have the same performance impact as turning with the same angle of bank in a non-moving airmass". That is absolutely factual. All that differs between flying in a constant moving airmass (i.e. wind) versus flying in completely still conditions is the aircraft's track across the ground. Momentum has nothing to do with the aircraft's airspeed and performance and (in a "constant moving airmass" i.e. we're not talking about gusts, wind shear or turbulence) a turn downwind is absolutely equal to a turn upwind and involves no difference to airspeed.
Accident's of this nature can often be attributed in part to the actions of the pilot in regards to their perception of movement over the terrain, especially at such a low level. In a turn from any direction whilst travelling downwind the aircraft is going to appear to be not changing its course rapidly enough (in reference to its position over the ground) and this is often when a pilot starts pulling excessive levels of bank to correct and attempt to achieve the desired turn radius. This accident is probably very similar to many one reads about where a pilot's turning from base to final on approach with a strong cross-wind that's causing them to overshoot during the turn. There have probably been hundreds, if not thousands of these accidents since flying started. Pilot overshoots turn, and banks further and further trying to stop the overshoot then, with their airspeed already low for the approach and their angle of attack high, they stall and spin in.
What's particularly disheartening is that it's clear from the many photographs and videos no flaps were deployed. On a 40º+ day, which (as a previous poster has noted) would have involved a density altitude in the thousands, to be flying low and slow in a strong wind (in a relatively large aircraft for the "box" it needed to remain in) and to be attempting to do that without the extra lift flaps would have provided does seem to be asking for trouble.
My condolences to the friends and families of the pilot and his passenger too. Having spent some years operating a floating hull aircraft myself and having spent a lot of time flying low and slow down close to the water, I know just how easily this type of accident can happen. "But for the grace of God...."