If you can get hold of the receipt from Jabba, it's deffo worth giving them a call.
Once that valve seat dropped out (I suspect that's the route cause - hence why you've got a headless valve in no.4) the cam belt will've skipped as Yoof said, which is when the other piston/valve contact occurred.
You've been bloody unlucky with engines, and sadly with the head it's not really something you'd have a hope in hell of noticing when building it.
I was telling you in the car about the same happening to my friend Marshall's track car - he had a moody head that dropped a valve seat and stuck a hole in the piston. The motor had seen 8,000rpm regularly, but he was lucky in that the valve seat dropped out whilst cranking it over - so it holed the piston but didn't damage the bore.
See what Jabba say - your best bet is probably to try to salvage your melted piston block. Is the bore badly damaged on that?