Math - "Oakmaiden, why not ask the children who have read it before to make sure they can keep the secret, and make it some sort of game?" Obviously - doesn't work with every child though.
"Would you not see if there was an alternative to having those who knew the ending spoiling it for everyone else? Would you think it might be worth your while to give out a small bribe prize to the children who can resist the temptation?" Seriously? Every time I want to tell a story to a group of children I have to bribe them all to not tell the story to others? I have a better idea - how about I just remind them of the expectation that they listen whilst I read, and prompt them again when they forget?
"Having them sit together as a team and hold hands as a reminder not to speak during the story?" Um - almost guaranteed to have them squirming and giggling together, I would have thought.
"Or maybe consider confining the reading to books no-one has heard before (harder to do)." Almost impossible to find good quality books that NO-ONE in the class has had read to them
"Just genuinely curious about the strategies you have in place for this sort of fairly predictable incident, given that children are inclined to relate things they encounter to themselves and their lives, and that they like to 'share with the class'?" Yes - there is an expectation that they listen quietly. Obviously there are regular points during a story where "audience participation" is encouraged, and as a class we discuss what might happen next, but children do know that shouting out is not welcomed. Sometimes children forget, or get over excited. Then they are gently reminded. If a child is being persistently disruptive I might ask them to move to sit on a different part of the carpet (next to me, normally). Gosh - does this sound a lot like what may well have happened to the OPs child?
"I don't think asking for comments beforehand is going to affect the likelihood of them chiming in with comments during the story." Well, I seem to recall that you started off your argument on this thread by suggesting that if the teacher had done just that then all problems would have been avoided. I quote: "She should have invited comments about the book before she started, maybe asking who has heard the story before, who has it at home, what do they think the story is about based on the front cover, who do they think the main character is, based on the cover, etc. That way the children would have been drawn into it and all the chit chat would have been dispensed with."