Hi. Thanks for using JA! I will try and help you out.
It is not possible for fuel to get into the oil from an injector. The High pressure oil is much higher than the fuel so the oil will always go into the fuel.
Since you have had the head check for this problem, the only thing left is the fuel transfer pump. It is located on the back of the HEUI pump. I would recommend replacing the pump.
Please let me know if you have more questions.