Speculation is a bit ridiculous. I think it is easy enough to remove candidates but there are still a decent number of people left. The only ones that I think we can 100% rule out are Ron and Aurora. we can probably rule out Aubrey because Julia discusses some other issues that the two had. Outside of that, I can't say if it was Eric, Joe, Julie, Gavin, or Victoria.
And I suspect that who ever it was was was more an idiot then a racist.
It sounds like one of the reasons Julia didn't appear in the first few episodes may be because of the racism angle, there was too much conversation about the two incidents that she mentioned and TPTB were doing their damndest to pretend it didn't happen. It might also explain some of the tribe divides.
I would actually argue that it might have been Eric or Julie because he was tightly allied with Ron and I can't see Ron allying with someone who was using a racial slur.
Overall, the season sucked for a variety of reasons and now we have another reason why it sucked.