IW on Free Throw

[Nevadaref]

You are correct on both the mechanic and the ruling after the whistle.
Here are the rule citations you requested:

1. According to the NFHS Officials Manual on page 41:
#241 For all free throws, the Trail official will be responsible for:
...
c. Noting whether the ball goes through, contacts, or misses the ring.

2. After an accidental whistle, the ball is put back in play by using the POI rule (4-36)
RULE 4
SECTION 36 POINT OF INTERRUPTION
ART. 1 . . . Method of resuming play due to an official's accidental whistle, an interrupted game, as in 5-4-3, a correctable error, as in 2-10-6, a double personal, double technical or simultaneous foul, as in 4-19-8 and 4-19-10.
ART. 2 …Play shall be resumed by one of the following:
a. A throw-in to the team that was in control at a spot nearest to where the ball was located when the stoppage occurred.
b. A free throw or a throw-in when the stoppage occurred during this activity or if a team is entitled to such.
c. An alternating-possession throw-in when the point of interruption is such that neither team is in control and no goal, infraction, nor end of quarter/extra period is involved.