I agree with the bulk of your post but I do think you are misrepresenting his opinion somewhat - he does agree that on snow, winter tyres are better.
There are an awful lot of factors that make a difference other than temperature though - water clearing ability of UHP tyres are often better than mid range tyres for example and that doesn't just go away when the temp drops...
Totally this. I have no problem accepting the clear advantages of the correct tyres on snow and ice and even specifically recommended them for merlins situation given the huge weighting he placed on snow performance.