I posted this on NFHS board, and got little intelligence for responses. Thanks to BktBllRef for directing me here.

PLAY:
Timeout called by A.
Team B has legal authorized conference near sideline.
During conference B2 is replaced by B12.
Coach decides to go "Goalline Defense"
B2 returns to conference, B12 returns to bench.

My Ruling:
Illegal Substitution (but prefer to use preventive officiating and inform coach B2 is ineligible for this play,
and that B12 must play this play)

Exception: B can buy B2 back into the game with an additional timeout, or B2 becomes eligible again if a dead ball foul occurs (not possible, at this point, for the period to end or a penalty from the previous play to be accepted during this dead ball period)

Agree/Disagree/additional thoughts or insights???