First, I am a tire engineer and I have a certain amount of insight into the engineering of tires as well as the way some of the industry statements are written.
First, this is an unusual situation. We're talking about old, but unused winter tires.
Most of the industry statements on the subject of shelf life deal with regular tires with the expectation that they will be used for many years after purchase.
Most industry statements on tire age are geared to address extremely hot climates.
So if we are talking about parts of Canada where it's really cold, I think those "guidelines" should be taken with a grain of salt.
I also think if we talk about 2 winter seasons of use, then we have to consider that the tires would be 10 years old - which is an acceptable limit for cold climates.
I also think that if a lot of attention is paid to when the tires go on and come off, then those tires could be used successfully.