You are correct, the circuit's neutral attaches to the AFCI break and the AFCI breaker has it's own neutral that attaches to the neutral bar. The problem arises in that many times the circuit may be sharing a neutral with another circuit. You'll know if it is or not by following the hot wire back to where it enters the panel and seeing if there is a red wire also in that same romex sheath, or if the wire itself is red. If so, the AFCI breaker will not work correctly and new wiring will have to be ran to feed the circuit. The wire length isn't a problem though. It's not ideal but you can splice the wire to lengthen it if needed. In my experience, AFCI's can be very touchy. Let's face it, not all electricians are very good. Sometimes wires get stapled down a little to tight and sometimes connections are not made very well. Both of these can cause an AFCI to trip, but under normal circumstances are not that big of a fire hazard. I know it sounds bad and it is, but you would be amazed what passes inspections and the general quality of work these days. As a former electrician, I've seen enough that if I ever build a house, I'm doing the electrical work myself.