Zane and Straff meet with Penrod in the night
We haven't had a Straff Viewpoint in a while, though from here out they get a little more frequent. Mostly, I had to include this to let you know why the merchants switched sides, and to give you a little hint of what was going on behind the scenes in places the heroes couldn't see.
I also wanted to remind you of Zane's penchant for poisoning his father, and Straff's own use of that mistress to heal him. This entire plot cycle (with the poisoning) was a late addition to the book in the revision process, added to give more dynamic between Zane and his father.
The really funny thing about all of this posturing, searching, and threatening in order to get the atium is this: atium is worthless. Or, rather, it only has worth as long as people give it worth.
In the minds of all of the characters, this cache is a fabulous treasure. Don't judge them too harshly–think how hard it would for you to pass up gold or diamonds, even if you were in the middle of a catastrophe. That's what's going on here. They still see atium as being incredibly valuable, even though the truth is that it was only valuable because the Lord Ruler made it so much a foundation of his economy.
True, atium can be used by Mistborn to do some pretty amazing things. However, you don't need a whole cache for that. Zane has proven that he has enough atium to kill Vin if he wants, and so more really isn't necessary for the Ventures.
Another worry, however, is that there enemies will get it–and that will let the enemies use their Mistborn to assassinate without as much fear of repercussion.