I have not yet engaged in any of the measurement protocols you are discussing about this project; but, I have extensively researched many "MB concepts unfamiliar to me," with this being the foremost among them.
It is essential to note that harmonic balancers also decay. The rubber allows the degree wheel to shift slightly, resulting in inaccurate readings.
Confirm the accuracy of the HB by measuring TDC in both directions using the 'long item' inserted into the spark plug hole, and thereafter calculate the average of the two measurements.
It seems to me that 18 degrees would indicate improper functioning, even if it pertains to just one bank.