A single 10% overcharge round might do nothing more than bend or warp locking luck a little bit or make a hairline fracture worse, but not cause a catastrophic failure. The integrity of the metal should probably be verified first if we're being extra safe. THEN you can trust a proof load.
Well, if we believe the OP, he already had it magna fluxed and hardness tested in key areas. So it had not inherent or incipient cracks and was still surface hard. I'd have proofed it, personally, after the above tests. I'd headspace before and after to see if there was perceptible lug setback after the proof.
It's academic in any event. I hope the new owner sees this thread before getting potentially hurt.