Correct me if I am wrong, but 850TT/25ME isn't really just shy when even those regionals hiring want 1000-2000 TT (which, true, you are sort of shy) but 200-500 hours of multi?
I would say stick with the king air job because you are getting multi time (good multi time too, fun!) and it beats renting a 1960s seminole. You are getting close on total time, but nowhere near where the multi time needs to be.
As others have said, maybe you can get a 135 freight operation. That will get you the multi time and the cross country time needed for the new ATP requirement.
Quick side (hope to not hijack), if it is say a king air 90 (below 12,500) on a part 91 flight and you are the sole manipulator of the controls, can't you log it as PIC multi turbine time?