That is unnecessarily complicated. You can easily get a mM7 chord on any diatonic scale degree by simply switching the key to the corresponding relative mode and borrowing the tonic seventh chord from Harmonic Minor. So to get a cmM7 in Bb Major you just switch the key to C Dorian and borrow cmM7 from Harmonic Minor. Also in which universe is D a fifth up from C?
You can access mM7 chords on non-diatonic scale degrees by switching the key to any parallel mode that has this scale degree and then repeating the process above. So to get a cbmM7 (which is enharmonically equivalent to bmM7) in Bb Major, you switch the key to A# Phrygian, then B Lydian and borrow bmM7 from Harmonic Minor.