I was working from the OP and that was not involved, however I am still 99 out of a hundred times calling this incidental contact, unless it is excessive, Flagrant or intentional. becuase if the player is out OOB they are not a part of the play at that moment, because they are not on the floor.

So now you answer the question
Assume the basket goes in and the colission knocks B to the floor, it is a Dead ball and B cannot get the ball to inbound it, are you going to make the foul call with contact during a dead ball?

It was part of the OP, I just made it more specific.
If it's a foul you'd call with the player in bounds, it's a foul when he's out of bounds (assuming LGP isn't an issue)

It doesn't matter if B can get to the ball, it's incidental unless it's intentional or flagrant. Since the ball is dead, this is easy. The OP was very clear on asking about if the ball was still live.

Now, if the collision happened before the basket went in, I'd treat it like rebounding contact. If A3 knocks B2 to the floor in rebounding action prior to the ball going in, do you call it?

If B2 had an oportunity to be involved in the play - in other words if the ball came off the rim in an area where possibly B could have gotten a rebound had they not been knocked down I may have a foul call. However if the ball was to the left and B is to the right side I'm probably going to rule the contact incidental since it has no bering on the play on the ball.

it doesn't have to have a bearing on the "play on the ball," IMO. What if B is the point guard and his teammate is looking to pass to him now? Displacement does not really require an obvious advantage in this situation. Normal pushing and shoving? Okay, I'm with you. Knocking an opponent down during a live ball needs to be called, though.