In the small bar setup with FOH speakers if you draw a line between the two FOH and the Floor monitors are on the line facing the other direction you will also see the same polarity problem. FOH push out and Monitor Push out but opposite directions. Causing a Polarity issue. Reversing the Monitors polarity makes the monitors lower registers audible and removes the polarity cancellation.
No.
It does not matter what direction a speaker is facing, positive pressure is positive pressure. There will always be phase differences between multiple speakers on stage due to the arrival time offset caused by differing distances between the source and the listener, but that is not polarity, and back to back speakers do not cancel each other out.
Mac