[Back In The Saddle]

You're within the time frame to correct the error ("no later than during the first dead ball after the clock has properly started").

"If the error is...the awarding of an unmerited free throw, the free throw and the activity during it, other than unsporting, flagrant, intentional or technical fouls, shall be canceled." As you point out, there is no bucket to cancel. The unmerited free throw ended when it became "certain the try will not be successful". Anything that happened after that is part of a new play, not "activity during it".

"Points scored, consumed time and additional activity, which may occur prior to the recognition of an error, shall not be nullified." So the subsequent basket by A is not canceled. So put the ball in play by a throw-in by B anywhere along the end line, and go on from there.