Uilos

In Final Empire, Kelsier tells Vin to burn any excess metals in her system before she sleeps, noting the toxicity. It does exist, but the idea is that the metals aren't staying in the system long enough for their to be an effect. There is also the allows tidally sound versions of metals, mixed wrong, the metals become toxic, though possibly not lethally. Vin had gotten sick many times trying to nail down duralumin.

Well, Marsh DID say that Wax was doing his Brother's work.

There is a lot being said in that chapter. The limitations of Sazed being primarily among them. There is also something else that was deeply discussed: Hatred. Wax's Hatred of Harmony and of Himself. While it is the first time the two speak after Bleeder's identity is revealed, it seems like such a marked thing to say when a Red Haze is shown invading, and one of the Main Villains is a being with Red Eyes. I think that the Red Haze is the Power of Odium breaking in. Harmony is a danger to Odium, being the first time two Shards have been united/held by the same person. Of the Shardholders, Sazed is still relatively inexperienced. Odium may be trying to get rid of him before he can exert his power more externally.