You mentioned not being able to test the motherboard with a different CPU. Maybe you should buy a cheapie one like a 3200 or something just to seel if it would boot without a CMOS reset. Since the CPU performs the Power On Self Test, if there was a problem with it, it would either recognise its own problem, or it would be unable to finish the test due to that problem, or both.
Possibly, upon CMOS reset, a full POST is not performed. Maybe the POST after a reset is only partial. If we assume this, that would explain why the CPU boots into windows but doesn't POST. It may be a problem that doesn't have anything to do with the actual performance of the CPU, it may be just something residual from factory testing that is now broken.
Having exhausted every other option besides changing CPU and motherboard supplier, I suggest you try these 2 things.