I agree that it sounds like the g slot. But I also agree that it can't hurt to check and tinker with the relief -- you can always reset the relief back to where you started, so it doesn't really hurt. Note, the 3rd fret/1st fret thing only helps identify if the nut slots are the right depth, not relief. I wonder if you've done a lot of tuning and retuning of the g string. That would make it act like a file in that groove, and could explain why you have grooving on the HEADSTOCK side that's deeper than the neck side -- if it were caused by playing, I'd expect the deeper groove to be on the neck side. CA and baking soda works great -- I have also used CA and ebony dust to adjust the slot in an ebony nut I made -- it's a neat process, but messy -- be sure to tape off the area, and work with small small small amounts. If there isn't already a tip/guide thread around here, I can post my experience.