I'm afraid I'm unable to help on why the tyres are worn on the edge - but your conclusion that the tracking is out sounds reasonable.
In terms of the tyres cracking, leaving a car to stand still for long periods with no use can certainly damage them, but I've never heard of underuse being a reason for general surface cracking (I will defer to others on this).
However, I can tell you that all tyres have a lifespan (irrespective of mileage) and will eventually begin to crack with age. This is why every single tyre has a DOT number on it which reveals the factory code and production date (the last four digits - the first two digits stand for the production week and the last two indicate the year).
According to Protyres, the lifespan of tyres is as follows:-
"If they are looked after, a car tyre has a “shelf life” of up to 10 years. Again, this is just an estimate because every brand is constructed from slightly different compounds. More important are the conditions the tyre is exposed to. If you literally keep a tyre on a shelf, protected from air, dampness, light and temperature fluctuations, it will be good to drive for at least 10 years. If left on an unused vehicle, they will be damaged in a matter of months if you don’t jack up or rotate the wheels. On the road, tyres need scrutinising after 5 years of age."
In light of the above, my own position is that I replace any tyres when they exceed 5 years old and exhibit multiple surface cracks. If there is no cracking at all, I would keep a tyre until it reaches 10 years old.
The original set of Michelin Energy Saver tyres on my car were produced in May 2019 and began to exhibit light surface cracks after 4 years (my car is not garaged), but I kept using them for another year before replacement (all at 30,000 miles).
