1-wire interfacing with motor (design suggestion and review)
Should I use this protocol even though it is slower than I2C?


That depends. How often do you need to send a new value to the motor? Is there a minimum latency? Is this safety-critical?


  Is it reliable?


Provided you follow the guidelines, yes.


  How do I interface a normal device (motor) to the 1-wire bus? Are there any modules I can use for it?


You'll probably need to build such a thing from a microcontroller.

Depending on the size of your motor, you may get better results with DCC - it's designed specifically to control the 12V motors used in model trains.