A scores.

B heaves a throw in the length of the floor that touch's nothing! (Floor, BBoard, Player, net, rim... nothing except padded wall on other end)

Goes out far end of the court.

Where is A's throw in?

It's at the spot that B had when they threw the ball. If the ball hits nothing, it is an inbounding violation on B for not throwing the ball directly onto the court. The violation does not occur where the ball hits OOB, but at the spot it was released.

BTW - team A does not get to run the endline :)

A player shall not violate the following provisions governing the throw-in. The thrower shall not:
Rule 9-2-2 ... Fail to pass the ball directly into the court from out-of-bound so it touches or is touched by another player (inbounds or out of bounds) on the court before going out of bounds untouched.

Therefore, the player making the throw-in has violated and the ball will be awarded to the opposing team for a designated-spot throw-in from where the ball was thrown.

See Penalty Section 2.