The only unanswered question is whether the "commercial panel" will accept CH or BR breakers. To my knowledge, Cutler Hammer doesn't make their AFCI in other breaker types.
Re-pulling would ba a drag. It is 160 feet of PVC with many bends. I would have to pull the whole mess out of the pipe and re-pull with the second nuetral conductor into an already over-loaded pipe.
When you say long and skinny do you mean 160' x 29'? Is the other end of the conduit on the opposite end of the building from the panel, or is the house longer than 160'?