Well, lets look at this.
You recabled your K701 with a double-entry cable at some point. We know this because you later sold the cable here. Since this requires soldering to both drivers, I can't think of a way that this wouldn't void your warranty.
I mean, if you sent 'em in for an issue with the headband, you could argue that your recable job had no effect on the headband. But you had a driver go out.
I've soldered many AKG drivers. The way they attach the coil leads, you could have easily damaged one of them. In a way that's often fixable - but AKG isn't gonna check that. And you didn't either.
If i were AKG, I'd have charged you for the driver and for labor installing the driver and reinstalling your stock cable.
If i were you, I'd stick to the issues of why they thought you needed a new cable and how they managed to send it back with so much damage to the frame.
They aren't going to budge on the driver issue, and i don't blame them. I wouldn't either.