The two basic problems that I've run into with air/liquid heat exchangers both have to do with flow - either the air or the liquid is moving too slow (too fast is also a possibility, but not very common and certainly not your problem).
You seem to have good air flow (although your testing methods are not very reliable). Low flow is usually caused by a bad pump or blockage (stuck 'stat, something in a passage, heat exchanger pluggage, etc).
You've checked all the easy stuff. Go ahead and pull the pump for inspection. Use your borescope on the block while you have things apart.
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