In my opinion, the producers set the contestants up.
They were using domestic freezers - these would have been struggling to get down to temperature, in the sweltering Bake-Off tent anyway. They needed blast chillers, like the ones the contestants get to use on Australian Masterchef.
They didn't have enough freezer space for all the contestants' creations - why else did Ian have to put his in the other contestants' freezer?
The producers had to know that inadequate freezer capacity and temperature was going to cause melty alaskas, and stress amongst at least some of the contestants - I think they were banking on that. The whole thing with Diana was just a bonus.
I also suspect that, because of the woefully inadequate freezers, Ian's icecream was pretty soft when Diana took it out - it explains, for me, how it got so soft when it was only out for 40 seconds.
I have to say too, that I was a bit
about Ian's icecream anyway - it might have tasted delicious - but grey icecream with black bits in? Never going to win any beauty contests!