Wanted to follow-up. I took my jacket back to Concord a final time, and they offered to replace the zipper. Since it's a complicated seam, I was nervous about having them do this and wanted to know exactly where they'd be sending the coat. The employee offered to have the manager call me with more details, so I left the coat with him with a promise not to do anything until they had talked to me.
The manager didn't call, so I called back a few days later and was told that they could try to fix the zipper again--it had probably just been done improperly the first time. I agreed to this, and they fixed the zipper, which seems to be working well now.
I'll probably give them another go since my clothes came back clean and unstained, but I'll be very wary about sending them anything zippered again!