I am thinking two things as to why the artifact end up on Rivendell's mountain. It might be the last place Lizzie was able to go to before she lost her memories. Or Rivendell could be more sinister than it appears. Remember what Pixlriffs said about old powers coming back? They could have a hand in that. Whether intentionally or not.
regarding the first theory: i can see your reasoning, according to Pixlriff's map of the empires, Rivendell is one of the farthest empires from the Ocean Empire as it's at the Southernmost end while the ocean is in the North, but it wouldn't make sense for Lizzie to be the one to hide away the item that would restore her own memories in a place where she can't access it easily, so i doubt she was the one to put it there
i personally feel like the artifact was hidden there for sinister reasons, i'm just not sure who exactly to pin those reasons on. yes, the artifact was hidden in Rivendell, but that doesn't necessarily mean that they are the ones who placed it there. it's entirely possible that they are innocent and being used as a scapegoat for someone else who wants to harm the Ocean Empire or start a rift between those empires, but we can’t say for sure!
the Ocean Empire would be right to be suspicious of Rivendell for having the ocean orb, but i don't think we know enough about the situation to seriously accuse them of anything at the moment








