Calderis

@Kurkistan found this WoB a few days ago, and I felt it needed to be shared here. It answers all of the issues. It appears that Brandon had considered all of these things and either took them for granted in the writing, or they were edited out.
So there are counterweights on bridges that just aren't mentioned properly in the text. 

This is what we've been trying to explain all along. It stores innate investiture. As in your spiritweb. 
So a person who has nicrosil Feruchemy, and any other power, be it Allomantic, or Feruchemical, can store their ability to use that power in the nicrosilmind. 
They also need to have stored Identity to make that power usable to someone other than themselves. 
This still doesn't explain how a non soul bearer Ferrings can initially tap the nicrosilmind and grant the additional powers. Nicrosil is a known allomantic metal. The misting is called a nicroburst. If nicrosil inherently allowed Feruchemy in and of itself, every nicroburst would be a compounder, and this would have been discovered already. 
We're still missing a piece of the puzzle. 

I'll try to find the WoB, but there's one that says you could become an Elantrian via hemalurgy, but it would take multiple spikes. 
I'm guessing one of those spikes would have to be the connection to Arelon due to the nature of being Elantrian. 
So I give it a tentative yes.
Edit: got it. 
http://www.theoryland.com/intvmain.php?i=1096
If you can do it for AonDor, you should be able to do for any of the Selish Magics. 

You can find sample chapters of Liar of Partinel. Brandon actually advises against people reading them though. As far as I know the only way to read Dragonsteel is to check it out from BYU's library. 
And I as far as Dragonsteel is concerned, I'm pretty sure the few who have read it, aren't actually allowed to talk about it. 
Most references I've seen to Dragonsteel just consist of "happens pre-Shattering and tells the story leading up to that event."
I think that's about as much as we're going to get until it's rewritten. The few bits I've heard beyond that say that the story that existed has been canabalized into the canon books so much, that what does get rewritten is going to be a very different story, and we're probably better off not reading it. 

As far as I know, we don't know what caused the earthquake, but it definitely was not Odium. The Dor as we know it exists because of the splinterings of D&D and their power being forced into the Cognitive Realm.
The Dor as we know it was created by that process and is why the magics on Sel are location dependant and all nonSelish magics aren't. 
This means that AonDor, Dahkor, etc. All formed after the Dor was created.
Odium has been trapped on Roshar for at minimum 4500 years from the current point in SA, as we know he was there prior to the last desolation. 
I'm not sure of the exact time line, but Elantris has been said to happen hundreds and not thousands of years before the mistborn trilogy. Wax and Wayne happens roughly between the first half and last half of SA, so Odium splintered D&D and left at least a couple thousand years before the events we see, in Elantris.