[Idhomustang]

Could be a couple of things wrong, you need to run a leak down test and see if you have a burn't valve or bad rings. You can by a leakdown tester from many sources. I think Jegs sells them or even Summit. Maybe even a speciality tool supply store might have one.
Pull the exhaust manifold on the side that you are expecting trouble on and the valve cover. Pull the plug and rolled end over so that the cylinder in question has both intake and exhaust valves closed then add air. If it doesn't hold air, you have a valve problem if it does hold air go to the next one.
What did the pushrod look like? Bent broken?
You should have already called your engine builder, and break the bad news to him/her. They may not want you to screw with it at all, doing so might void the warranty.
But, it does sound like a valve train problem maybe several, bent valve, stuck valve, bent pushrod, just to name a few, maybe worse. Not having antifreeze in the oil, is just one of many things that might have happend the same could be said for gas in the oil.
I would call your engine builder, it sounds like something was wrong on his end espically if you have a loose rocker arm, did you assemble or did they? These should have torqued down to the proper spec.

Mac