Good afternoon Nigel.
I am no expert in snow(0r anything for that matter) however , thoughts are the ravine 4 was a snow collapse after A) finding a natural cornice /snow cave over the ravine( ravine meaning , between a couple or raised banks, and a place for snow to drift/gather) or B) they dug their own hole in to the snow drift, possibly from where the snow den was situated. ( I have read the statement that said it was little more than arms length ,from den to bodies in ravine and the distance had been exaggerated to 6 meters)
Regarding snow,the temperature would flux and warm fronts would come followed by cold fronts. How mild or warm I don't. I still don't know what firn snow is by definition, and here's my point, are they saying firn snow as the opposite to soft or fresh snow. Firn snow is snow that's been lying , maybe heated a little by the sun and fluctuating temperature for a long time. It usually means snow that's lasted a year.
I know you are talking about the snow over the tent. My guess and that's all it is, would be they meant by communication that the snow over the tent is firn /firm. It is not "fresh "snow. It has gone through some processes, heat, wind , cooling. I don't know if the wind blowing the snow in its little particles would sufficiently raise temperatures to cause slight melting and hardening of the snow.( I was thinking of the tree bark getting blasted off the trees through the winter season. No real mystery there but it gives an example of the wind direction and how it can erode things , like the boot rock)
In the UK I'm sure you have experienced the powder dry stuff that we can't make a snowball with or the perfect ,just right snow for a snow man. As I understand it , these lairs of snow fall, melt ,etc on all places/ mountain sides. That's why a lot of snow slabs or avalanches occur( I think) it's the different make up or densities of the snow. There's no big secret to it. If the snow got wet and slush like, I think there might have seen signs of it in the tent, and on the bodies or clothing. Saying that I'm not against some snow melting or steam by an orb or electro field or even a self destructing rocket that vaporizers it's fuel in a flash over a mountain side. Its just low down my list of theory choices. ATM. I am happy to be convinced otherwise.
Back to the hard snow at the ravine, I think, and I could really be wrong here . The pick axe's , shovels and six strong men were called to dig the ravine 4 out. The snow was very hard there which leads to the idea of there being a snow collapse. When the snow collapses under the right conditions and depending on the humidity or make up of the snow it will become compacted, maybe heat up then freeze. Not like an ice block but hard. again going back to snow balls , if there is a suitable thaw and you make a snowball out of slush it becomes hard and dense. And you don't want that being thrown at your head.
I'm not disagreeing with you on anything mate. Just trying to explain my limited understanding and willing to learn along the way.
So for simple snow terminology
Fresh snow= fresh snow
Firn = snow that's been lying about at been exposed to weather and temperature fluctuations , movement.