The wear pattern on the valve tip looks pretty close to perfect.
If the push rod is rubbing on the head, that's the problem. As it goes up, it hits the head, and pushes the rocker arm foreward. The stud is not really designed for lateral strength, and it snapped.
Since you're using an offshore knock off head, I would think the the hardware on them would be suspect.
You need to remove the heads to grind that area for clearance. That is a common problem/issue with any head, and one of the areas that needs to be checked during assembly. I would also replace all of the studs with good ARP studs. Be sure and use thread sealant during installation. And I would replace all of the push rods. The rocker arms, guide plates, springs, and valves should be fine.
Since this is your first engine build, don't beat yourself up over it. Good decisions come from experiance. Experiance comes from bad decisions. This mistake won't cost you that much. Some of the mistakes I'v made..........
