Hemalurgically, atium steals Allomantic Temporal Powers. But, that seems unlikely, since atium is a god metal. It wouldn’t fit in with the rest of the magic system. Did Preservation, in addition to switching Cadmium and Bendalloy for Atium and Malatium, also switch atium’s Feruchemical and Hemalurgic powers with Cadmium? Because it seems to me there’s not a lot of atium Marsh can use to live for hundreds of years into the next Mistborn trilogy.
Preservation wanted Atium and Malatium to be of use to the people, as he recognized that it would be a very powerful tool—and that using it up could help defeat Ruin. But he also recognized that sixteen was a mythological important number, and felt it would make the best sign for his followers. So he took out the most unlikely (difficult to make and use) metals for his sign to his followers. But that doesn’t have much to do with Hemalurgy’s use here.
Remember that the tables—and the ars Arcanum—are ‘in world’ creations. (Or, at least, in-universe.) The knowledge represented in them is as people understand it, and can always have flaws. That was the case with having atium on the table in the first place, and that was the case with people (specifically the Inquisitors) trying to figure out what atium did Hemalurgically.
Their experiments (very expensive ones) are what determined that atium (which they thought was just one of the sixteen metals) granted the Allomantic Temporal powers. What they didn’t realize is that atium (used correctly) could steal ANY of the powers. Think of it as a wild card. With the right knowledge, you could use it to mimic any other spike. It works far better than other spikes as well.
As for Marsh, he’s got a whole bag of atium (taken off of the Kandra who was going to try to sell it.) So he’s all right for quite a while. A small bead used right can reverse age someone back to their childhood.
But this was a little beyond their magical understanding at the time.
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