Yes, the disambiguating stuff involves the existence of other pieces. Yes, BigDoggProblem got it correct. (The bottom-right part apparently doesn't work as intended, but not a major issue there.)
OK, my bad. I might give it another try later.
EDIT: just to clarify. The order of preference for disambiguation is the usual file, rank, file and rank?
Yes, the disambiguating stuff involves the existence of other pieces. Yes, BigDoggProblem got it correct. (The bottom-right part apparently doesn't work as intended, but not a major issue there.)