1. With adhesive flat antennas you should mount them vertically as they have vertical polarisation.
2.To maximize signal quality I would recommend mounting them separated, so it's up to you either it is back and front of the car, or just left and right sides of the car.
3. There is none, it depends on the antenna and cable itself. It is practical to use as less length as possible because the longer the cable, the more significant packet loss there is.