How to resume after clock starts during throw-in that is untouched.

[bob jenkins]

Originally Posted by BillyMac

Point of interruption (one of four "magic" spots).

Although technically not a correctable error (although it's an error, and it's correctable), I'm pretty sure that 2-10-6 would apply:

2-10-6: If an error is corrected, play must be resumed from the point of interruption to rectify the error …

Use definite knowledge, a visual, or non-visual count, to correct the game clock.

5-10: ART. 1 The referee may correct an obvious mistake by the timer to start or stop the clock properly only when he/she has definite information relative to the time involved. The exact time observed by the official may be placed on the clock.
ART. 2 If the referee determines that the clock malfunctioned or was not started/stopped properly, or if the clock did not run, an official’s count or other official information may be used to make a correction.

Are you assuming that A touched the ball at some point during this?

As I read the OP, no one inbounds had touched the ball when the referee stopped play because the clock had improperly started.the