For the second question, both the P1 and the Adafruit board you mentioned already have an antenna designed into them. Are you trying to make your own board with the P0 and nrf8001 and have them share the same antenna?
For the third, yes, the bluz gateway will use the P0 and an nrf51822 on the same PCB. They will use the coexistence pins (to some degree). The nrf51822 doesn’t explicitly support the same type of interface, so there will be some other means to try and coordinate the transmissions between the two.