In the case of Gixer's Falkens (can't remember if they were 452 or 912), I have a theory that they were overdriven, which caused something to happen to them.
We might be getting somewhere there.. Might also apply to SC5s? .. although they were sh1te from day 1 with no hard driving? None at all?

You overheat them once or twice, either the surface rubber hardens permanently, or something in the tyre structure weakens, leading to a tyre that will never perform well. Other tyres handle the heat better, don't get as hot under the same conditions, or dissipate the heat better. SC5 are a fuel economy tyre, IIRC, which probably means less surface area on the road, so the rubber is stressed further, and has less surface to transfer heat to the road.
Smaller car simply can't get enough heat into the tyres?
I can't believe that there's something inherent in the Omega's chassis that makes the tyres perform quite as badly as you describe. Maybe they'd be better in a car with double wishbones, with less weight? My money is on them being sh1te full stop, engineered to get good EU ratings, or damaged by to6 much car.
It's irrelevant, because who, having read your accounts, would risk fitting SC5s to any car, given their cost?