From what I understand, water is it's densest at 39�F. Colder than that and it floats upwards. Warmer than that, and it does the same thing. But, in winter, normally the warmest water in the pond is the densest if you don't have water entering the pond.
I'd go with the siphon and not overthink it.
In the wintertime, you don't want to have your bottom diffuser on the bottom of the deepest part of the pond anyways, it should be around 1/3 to 1/4 the total pond depth so it will leave the densest, warmest water at the bottom of the pond for a "warm water refuge".
This is my take also. My biggest concern for our ponds involves RES.... the warmest water in the dead of winter will probably be at the bottom of the pond, @ 39 degrees. Maintaining adequate DO in that deep water would seem to me to be important for RES survival. It's my understanding that they are limited in their search to find oxygenated water by their inability to tolerate the colder temperatures associated with going shallower in the winter.....succumb to lack of DO, or perish due to cold water?
This may be an extreme example, unlikely to occur, but a pond in Texas is quite possibly a different animal from a pond in Ohio, during the winter. In the winter I like to concentrate my efforts on maintaining adequate DO, and leaving the water near the bottom untouched, and unmixed. It's worked that way in my ponds for over 40 years, with never a fishkill....so far, so good.
