[assignmentmaker]

The question: "A1's throw-in is touched by B1, who has one foot touching the
boundary line. Official rules this a throw-in violation and awards team B
the ball at the spot of the throw-in. Is the official correct?"

I believe these are the relevant citations.

4-42-5 says: "The throw-in ends when the passed ball touches, or is touched
by, an inbounds player other than the thrower."

9-2-2 says: " . . . The ball shall be passed by the thrower directly into
the court from out-of-bound so it touches or is touched by another player
(in bounds or out of bounds) on the court before going out of bounds
untouched."

9-2-10 says: "No player shall be out of bounds when he/she touches or is
touched by the ball after it has been released on a throw-in pass."

Under 4-42-5 the throw-in has not ended.

Under 9-2-2 A1 action is not a violation.

Under 9-2-10 B1 has violated.

Thus, in the question in question, ball to A1 at the spot of B1's violation.
AND, if the throw-in is an alternating possession throw-in, A keeps the
arrow.