It used to be quite common to wire a string of outlets on two circuits by making all the tops one, and all the bottoms another. The bars on the side of the outlet that connect the top and bottom would be broken off allowing for this type of setup, making each outlet a "split receptacle." Now, you usually would see a line of outlets on two circuits alternating, so every other one is in it's entirety, on a different circuit. The most obvious thing in your case would be that someone figured they needed a GFI in the bathroom, pulled the old split one and just stuck the GFI in because after all, the wires and colors in there matched the picture on the GFI box. (This surprisingly, might even sort of work sometimes if lucky, but I doubt the GFI would work right.) Next most likely is that another "split" outlet was replaced without breaking the side bar to separate the top and bottom, connecting the two circuits instead. Again, that this worked, if it ever did, is just mostly by luck. If I understand the whole thing correctly, I would cap the feed from 7 and put it back together from 9, which would make that outlet (only) a GFI. I assume it's at the end of the line because right now you're obviously not feeding any other outlets from that one GFI as you have two power ins and no power out, and apparently no other non working fixtures.
But again I don't advise you to just do that based on my assessment without seeing the whole situation. I'm not confident in telling someone to do what I just said unless they really understand what's going on. I don't know for example if you have the need to feed other outlets in that bathroom from this one. If you do, that's not the answer. It was interesting to think about the problem though, so I'm glad you posted. Love to hear how it all shakes out in the end.