Fast_digitalRead Fails to Compile

No, the attachInterrupt() function is not affected by the dual pin connections to pin52. The interrupt function will detect edges that are only a few hundred nanoseconds apart so any noise around the switching point could generate a lot of interrupts. I would suggest calling detachInterrupt(52) at the start of the service routine to stop subsequent calls to the interrupt from noise. At the end of the interrupt, you could then re-enable it with attachInterrupt(52). It might be a good idea to put a check in to make sure the pin is stable and in the expected state before re-attaching it!