I wouldn't rule the dirt out, but the OP did not mention any dirt in the throttlebody or ports. The are some very small vertical scratchs, but the cross hatch is still visible. The another tell tale sign of dirt ingestion is blackish grey goo from very small metal particles mixing with oil, don't see any of that.
On my cam timing theory, he did not have leakage past the valves when doing his leak check, so that doesn't hold water either. Like I said before nothing adds up. Gross low compression, but no obvious leak path.
KR, you said the valves looked good how did you verify this?