Yunyun's two appendices is not her only power.Consider how she seems to have rescued Maria from the exploded train car.
Impossible crossoversGirls with Guns who deal with the violent underbelly of the crime and government. Yes.
Synesthesia is a kind of Mystic EyeSynesthetics don't seem to inherit their eyes from their families, and the Ua Virus may just be another way of bringing those with latent abilities close enough to death to activate it.
Canaan's setting was created by a cosmic cataclysm when Nasuverse collided with Noirverse and Black Lagoon VerseSee both WMGs above.
Borners are an attempt at making artificial Contractors.The Ua Virus turns people into drug-dependent superhumans with drawbacks. The protagonist ''lacks'' aforementioned drawback. Clearly the virus was derived from Hell's Gate-related research :)