I understand you very well. Unfortunately I also have this problem with blender. I haven't found a solution but I discovered something after some time and many tests. If you exaggerate with the rotation of the neck and head (as in the photo below), in game the driver tends to assume an approximately acceptable position. It is as if the degrees of rotation between blender and AC are staggered.
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Previously his head was slightly tilted back and now the situation has more or less improved. The certain thing is that there is something wrong between the export of the driver_rigged and AC, I can't even manage the position of the fingers.