Hello Junkologist, I can understand your frustration with issues with a brand new engine. I don't know what you plan to do with it, but my first thoughts are a valve stuck slightly open with the no compression issue, (so soon) it may need disassemble and a careful cleaning.
The sloppy rocker arm could also likely be repaired, but would need to see where the clearance issue is at.
Hope you get it going or maybe reach out to Jin?