A very interesting concept, although I would have though that a better description would be 2 phase control of a 1 phase motor?
But skimming over the code it would appear that the high speed interrupt Read_ADC_Results is where the limit is placed on 60Hz?
The value stored at FREQUENCY.
I see the Freq pot is centre off and fwd and rev rpm is either side of centre, maybe changing the code to read the the pot from 0 to 5v in one motor direction would help increase the frequency range?
A bit of experimentation would confirm.