Well, I finally did frog back to the knot and redid it as a Russian join.
What a PITA!!!! The cotton would not pull through at ALL if I left it at normal thickness. So I unwound a bit and cut away two plies, leaving two to work with. STILL a major pain to pull the plies back through themselves. The "thick" parts don't actually look too bad, but the actual join looks a bit like a slub. Can't get all of the plies to pull through evenly, so a bit of a loop was left on each end.
All in all, not the prettiest, but not very noticeable on this project. I will never try this method with cotton yarn again, though.
Next market bag, I'm buying a monster skein so there's no joins at all!! lol
Thanks again everyone for your suggestions!