I like what you have done with this. Interesting ideas.
As for the bolded, that, unfortunately, is incorrect. I think it was in the end of WoA, someone was talking about Sazed regarding how large he grew when tapping strength, unlike a Mistborn, who stays the same general size. I believe it was stated that as long as Sazed had the strength stored up, there was no limit to the amount of strength he could drain. It seemed to be the same thing with everything else as well (like weight...he can become as heavy as he wants, as long as he has enough stored.
Perhaps it could affect Feruchemical storage, then. That is, if you "fill" the Nicrosil while filling another metalmind, you don't store as much of an attribute as you lose. But if you then tap Nicrosil while storing something else, you store
more than you would have lost. This comes close to breaking the "rules" (because it means you can store weight and get strength, basically), but I don't really see any other "enhancement"-type function available that isn't even weirder.
A thought on Atium hemalurgy: As pirsquared suggested, a spike that steals "life" seems appropriate. However, all spikes steal "life" in a sense: the victim must die for the hemalurgy to work, and if the spikes are put into a certain type of creature (or corpse), they grant sentience. But perhaps an
atium spike could steal either Health or Youth (most logical, of course, would be for atium to "grant hemalurgy" the way lerasium grants allomancy, but
everyone can already use hemalurgy, so there'd be no point). The reason the Lord Ruler didn't do this could be that he didn't want to spike himself much more than he needed to, to prevent Ruin from taking over completely (although Ruin seemed to have a lot of control anyway) and that it would look too suspicious (and decidedly un-divine) if he had to kill a small child every 75 years or so to stay young. Much better to do the feruchemy/allomancy trick, so people don't start catching on.