Slot.it rear wheel grub screws too tight

I've just bought a new Slot.it Audi R8C (CA12C) which has had its rear wheel grub screws over-tightened in the factory. I've ruined three new Slot.it allen keys trying to loosen the screws, only have two undamaged allen keys left and don't want to ruin those. I need to loosen the grub screws to reposition the rear wheels, which have been fixed too far inboard at manufacture.

I've tried alternately gently heating and cooling the axle and wheel assembly, which succeeded in loosening one screw, but the other is still jammed tight. Any ideas how to undo it?