Inability to push on metals in somebody is a matter of strength that got misinterpreted into a categorical statement by early allomancers. It's HARDER to push on something already invested, or on something within another person. The relative level of difficulty is high enough that, like piercing copperclouds, it was assumed to be impossible.
It never was, though. The only thing needed to pierce a regular mistborn's coppercloud was a hemalurgic spike on a mistborn. Duralumin would have also done the job.
Same with pushes. You also see that in Vin's battle with The Lord Ruler in TFE. Vin feels like TLR is pushing on the metals inside her body, and that's because he actually is.
So a powerful mistborn like Vin, burning all she's got with Duralumin, was enough to push on metals inside the inquisitors.
Yes, a sufficiently powerful mistborn (Lerasium mistborn, powered up by hemalurgic spikes, and burning duralumin) might be able to just kill someone by ripping apart the metals in their body. I don't know how much power that would take (or maybe it would take enough strength that it's still impossible, there's nobody that could!) but in principle it's possible, just takes a LOT of power.
