So just to be clear, the suspect driver works fine except for only going in one direction and the symptoms are the same if you feed it from the breakout board outputs that work with other drivers, and the motor/slaving works if you use a different driver for C?

Is the driver heating up abnormally?

Assuming the above is right, the driver is ignoring the direction input. That implies the opto-isolator inside is broken (so the PGA isn't getting the signal), or something else which will be more difficult to fix. If you want to post it to me I can have a look at it and hopefully fix it, just send me the return postage.