What's the Difference Between Kelsey and Mistborn Secret History?

I think that our steel is at the crux of what is going on, not just with Kelsey, but also with the heralds on Roshar and a little bit of the mystery that we started to get with the end of Rhythm of War. I feel like what we know about our steel is that a bunch of breath was dumped into him in a similar way. And what we then saw from Kelsey in Mistborn Secret History is that that creates a cognitive shadow that is able to resist moving on into the beyond. But if his body was then given a command that turns him into a lifeless, someone alive on Nalthus is doing that. It's not necessarily his choice