What happens when Jyn's holster adroitness meets Greedo's tentacle fingered attempts to procure his firearm, but the app includes a caveat that Greedo gets an extra black dice. Here's how I resolved it, let me know if it is accurate:
Greedo activates. Jyn shoots because quick on the draw precedes anything else.
Jyn shoots again because of Greedo's slow on the draw. Greedo gets an extra black dice.
Greedo shoots, Jyn does not defend with her white dice due to the caveat of the app.