Your coolant probably looks a lot like chocolate milk right now.
Radiator blowing up....a quick guess would be you rapidly bled combustion gas into a coolant passage. Pressure build up blew the (presumably) plastic end tank. I'm surprised your coolant hoses didn't blow first. Of course, there are advantages to having a stock radiator cap with a stock radiator (not that that has anything to do with your motor seizing)
My advice, call the builder and calmly & professionally explain the issue and see what they can do for you.