Going lower in pot resistance makes the problem of end resistance worse, not better. In addition, most references top out at about 10ma of source current so a lower resistance pot is really not practical nor desired.
I'm getting 7.87 Vdc from S0 to S2 when it should be 10 vdc to come to a stop
That make no sense to me. Unidirectional convention is that zero volts is zero speed and +10 volts is max speed. What does 7.87 volts have to do with stopping or min speed?
I'm thinking if I add additional resistance to the S2 to S1 line it would send more to the brake (S0)
S0 is ground, it has nothing to do with a brake. Did you have this drive configuration running with analog pots first? If so, please post a drawing of how things are wired as this discussion is now at a stage where you statements are no longer matching up with the original description.