A couple things. First of all, Marsh wasn't Mistborn, he was just a Misting before the Inquisitors made him one of them. As for the creation of Inquisitors, it is hard to make a guess as to exactly what it entails. However, there are a couple points that haven't been mentioned. First of all, right before the Lord Ruler killed Kelsier in the first book, he pointed out that the Inquisitors are "very hard to replace." Second of all, when Marsh talks about the process of creating Inquisitors, he says that it is "messy." Based on all of the scenes of sacrifice that have been associated with Inquisitors, I think that it is safe to say that Inquisitors can't just be created by using metals, there is something much more sinister involved. And third, something I just noticed, Marsh said that Inquisitors have 11 spikes. So, when I look at that, and at "very hard to replace," it gives me a couple ideas. There are, in total, 16 metals, including the two that we haven't seen yet. The two that are unknown are theorized to be able to give or take away allomantic power. Duralumin and Aluminum both directly alter other allomantic metals. So, in theory, anybody who can use the other allomantic metals can use these. Or, perhaps Inquisitors can't use them. We haven't really seen. However, that leaves 12 remaining metals. Marsh already had skill with one allomantic metal. So, theoretically, each spike grants one allomantic power. If this is the case, then it would make sense that the spikes would be made of each of the allomantic metals. That would explain a lot. For example, since we know that Inquisitors do, in fact, die, it would help to explain where the Lord Ruler's atium cache went. If he had to use a railroad-spike-sized portion of atium to create each individual Inquisitor, that could take up a fair amount of atium. Or, perhaps all of the Inquisitor's spikes are made of atium for some reason, in which case, it explains it even more. It would explain "very hard to replace" certainly, as well as the atium shortage. However, I think that there is a lot about Hemalurgy that we still aren't seeing.