Your statement "nobody possessing the ball" means that team control was not established. Absent team control inbounds, you cannot have a backcourt violation.
Two more over and back questions:
Team A has possession, B1 taps the ball away from A1....A1 touches the ball last before going into the backcourt and then touches it first in the backcourt...Over and back?
Same situation as above except...A1 touches the ball last in the front court and A1's teammate (A2) is the player to retrieve the ball in the backcourt...Over and Back?