I'm working on a friend's 12s Keystone Howard, so I'm really trying not to break anything. This is the first Howard I've worked on. It looks to me like the stem needs to be removed before the movement can be removed from the case. Is this true and, if so, what's the secret?

I can see a small screw above the crown wheel. That is a detent screw for the stem and crown. Turn it out about 1-2 turns and that should release the crown.
In your photo it is the screw to the left of the case screw that has been removed.

Thanks, Aaron!