Yep return. Glad you got your SA to look for a new one. A stitch is not that big a deal, especially on the inside. However, not acceptable in a brand new bag.
Hoodie,
I'm not sure which NM did you go to, but I had the same problem with my GST and fortunately they are able to fix it on the spot and this is at NM in Austin TX (I opted for repair because they dont have the GST in the same color in stock). However, if you have option to exchange, I think it's better and faster.
Loose stitching is a common problem and an easy fix. HOWEVER since your bag is brand new, I would just wait & exchange rather than send it to repair.