Playing distance is not well defined but would be interpreted to be that the ball must be within the ability of the player to reach out and touch it with any part of his body that can legally play the ball.
Not necessarily, as players are allowed to protect possession of the ball if it's going out and can't reach it in the way you describe.
But that's all moot as the situation to me appears as incidental contact until Durante's arm intervenes.
The instructions explicitly say that "All players have a right to their position on the field of play, being in the way of an opponent is not the same as moving into the way of an opponent (i.e. it's not obstruction)". Culina was attacking the ball Durante headed away, and just happened to be in his way when the latter tried to go around him. Quite clearly, in reference with the LotG, Culina did nothing wrong. It was Durante who committed the foul by holding Culina's arm and moving him out of the way with his arm, which constitutes a push.
Admittedly, the contact wasn't heavy and Culina made the most of it, but it's still a foul under the LotG.