This does not inspire confidence
Serial.print(" LeftMSpeedF =");
Serial.println(RightMSpeed);
When I saw this
the left motor does not twitch UNTIL LeftMSpeed hits 255 in either forward or reverse direction. Then it kicks in and goes at full speed in whatever direction is required of it.
my first thought was that you have the driver pins wired incorrectly.
Another wild thought ...
You are using pin 10 for the PWM signal for the left motor and pin 10 is the Slave Select pin for SPI communication. You don't have to use it for SPI and, as required, you do have it set as OUTPUT. I don't know if its role in connection with SPI would affect its ability to do PWM but it might be worth trying a different PWM pin to see what happens.
...R