Blown head gasket, likely cause????

I disagree 100% on the oil contamination - in my experience it is unreasonable to assume that. If you don't see oil in the intake ports, then get a close up of the cylinder walls and post it. My money's on the wrong wall finish for the rings used and the result is rings that have not seated, and potentially never would. You may not see damage on the pistons from detonation, especailly on a decent forged material. You will more than likely see it on the rod bearings (the backsides) & the main cap mating surfaces. This thing really should come out and be closely inspected.