You can use a smaller power supply voltage but you do not have to go as low as 68VDC; you can run the G203V right up to 80VDC continuously as it is a soft limit. The actual drop dead voltage of the G203V is around 115VDC so our voltage rating takes ripple and line voltage fluctuations into account.
The current set resistor is not a precision part so you will be fine with those current set resistor ratings. The half amp intervals in the manual are just there for simplicity and you do not have to use those ratings. If you used the formula in the manual to calculate the current set resistor value you would get 31K for the 2.8A motor and 20K for the 2.1A motor. A resistor +/- 10% of this value will work just fine.
It looks like you have a handle on what you need and what you are doing. It is all that digital stuff that makes my head spin... ;-)