I can't imagine two mechanics both missing the obvious, but sure, why not. Assuming the cable is tightened already as far as it will go, then it is either the chinesium throttle housing, or your cable is worn out. If it was my Z1000, that cable would be stretched and busted for sure
I don't see the need to run a return throttle cable, as long as you keep the entire throttle assembly clean and lubed. The return cable is to close the throttle butterfly even in situations where some dust or dirt is gumming up the works. Unintended acceleration is great when you're ready to donate your organs.
Nice bike. The Z1000 is a beast