Did a pressure spike cause this valve body to crack in half?
It broke because of the weight of the expansion tank. Only 5 psi on bladder side means it was full of water and very heavy. You need to support your expansion tank. Pre charge should match static pressure. The pressure does not increase or get higher without expansion tank so "overpressure" is not the cause of the problem. Expansion tanks Don't make pressure. They slow its action. Pressure increases slower and decreases slower. A fully filled expansion tank with little pressure on bladder side can like weight 50 pounds.