After going back and forth on a lot of workflows and tools, I'm going to try to use Bohemian Coding's Sketch to build prototypes of a mobile app and create the interactions our remote team can view and critique in InVision. Being a small team on a shoestring budget (hey, who says public-good work doesn't pay?), cheap or free is a great feature set.
And Sketch seems to be maturing towards being an easier (relative, not absolute) tool, and the company is including a lot of assets that'll help with app design, including automatic export of assets to 1x, 2x, 3x, and so on.
Here's my highly guided stab at making a rather gaudy icon – the tutorial was crazy with ways to use four shadows to compensate for Sketch's lack of beveling tools.