MS can cause slowing down or complete failure of conduction in some of these nerve fibers, which would likely interfere with the ability of the CPG to generate smooth movements. This doesn't mean that movement is impossible, but it would likely require a much greater amount of conscious control efforts than what is typically considered necessary for CPG-produced stepping movements. I have AVM and I am paralyzed from the waist down. Even though I cannot move my lower limbs significantly. I have come across the studies with rats.
Some researchers are optimistic, but others have reservations.
I gather that research into spine and nerve repair have been going on for some time. Christopher Reeves is positive that there will be a cure in the foreseeable future. Depending on your answers to my earlier questions, this could mean that such efforts to move your limbs might, in fact, be helpful. As for your second question, believe it or not, we still don't know. As a result of that damage, it's thought that some connections onto nerves in your spinal cord that control leg muscles are lost.