How can strength training prior to a spinal cord injury (paralysis) affect recovery?

A good start. Having good musculature before a neurologic injury may aid in recovery as the availability of neuromuscular junction receptors may be more primed towards stimulation. Also, post injury atrophy may not be as bad, so again more muscle to work with.
Determination. Injury to the spinal cord can be minor or severe. Excellent athletes who have worked out regularly can end up severely damaged with minimal chance for recovery. But the will and determination to recover is what will get the best outcome. Appropriate rehabilitation guided by a good rehabilitation team will provide the best chance for good recovery.