It's most likely the base metal, due to the fact that there's different names for electrum-mistings (oracles) and atium-mistings. (seers) That's certainly not proof, though.
I'm not sure what your logic behind this statement is. Are you saying that because the names for the two misting types are different that they can't be connected? Well, even if my theory is right, Atium and Electrum would still be two different metals, just put together to get what we've observed.
I think that this would be something that Preservation would want to do. Raw Atium would be extremely powerful, so he'd want to diminish its power, as well as offering a power that would better fit into his plans. So he had Ruin's power manifest in a place where it would mingle with an already present metal to dilute its power.
It makes sense in my opinion, but I just need one more proof to make it official, and that's Brandon's confirmation.
I do want to know what a Atium Larasium Alloy would do, though.
And that's simply bizarre, douglas.