WRT keeping the secret and not blurting out the ending:
Teacher would start the reading session with the announcement that it was time for the children to come to the circle for reading. There would be teacher-led discussion of the book as I described. Then there would be a reminder that the story was about to start and a reminder about letting others hear the story. The children would be invited to tell the teacher how they were to behave as the story was being read.
Sometimes the teachers would stop and ask for comments from the children. The points would correspond to beginning, middle, end. The children were asked to recap what had happened and anticipate what might happen next. Even those who had indicated that they knew the story could contribute, and the teacher would ask 'Do you think Sam is right?' if Sam had revealed the plot. Some children would disagree, some would agree. At four years old, children believe in the tooth fairy and just because one child who knew the book told the ending didn't necessarily mean they would believe it or lose interest. The teacher would say 'Let's read on and find out if Sam is right' and draw their interest back in.
Holding hands, and hands in general:
One huge rule of the classrooms I have experience of was that children were to keep their hands to themselves. Proper use of hands was explained and modelled in sessions at the start of the year that were devoted to what keeping your hands to yourself looked like, and instructive scenarios were acted out. There were little songs and rhymes that were sung by the teacher about hands (and other aspects of classroom behaviour). This happened before any attention was given to work that could be considered remotely academic. One of the uses of hands was to raise them when you wanted to speak. This instruction and songs/rhymes were repeated every day initially and then every so often as time went on. Often a teacher would use a rhyme or song to call attention to inappropriate use of hands instead of a spoken reminder about what proper use of hands looked like.
Children who were fidgety, inclined to pick at their clothes and laces, undo their hair, pick noses, poke others, pull stray tufts from the circle carpet were given a lump of blutack to fiddle with during story time.
Holding hands at the behest of the teacher worked for the most part once the proper use of hands foundation was laid (and there were frequent reminders) and there wasn't pushing. When each page was turned the children who were keeping the secret and holding hands together were allowed to squeeze the hand they were holding. If hands were held properly each hand holder got some praise from the teacher at the end of the story.
Generally, the hand holding was used just for the purposes of reminding the persistent talkers not to talk out of turn, and not specifically to keep the plot from being ruined -- the teachers were able to get around this aspect of the problem of talking after all. It worked a bit like tying string around the finger.
The teachers had a selection of 'hands' to raise wordlessly when children started blurting things out without raising their hands and waiting to be called on. They ranged from a decorated rubber glove with cardboard inside to retain its shape to a big sports foam hand to a colourful mitten with a button face, all mounted on dowel rods. This sort of tactic (along with songs, rhymes and mime) reduced the likelihood of the children tuning out the teacher's speaking voice. It also kept the voice for the class activity, and made the children stop and think a bit about what the instruction might be.
The overly chatty offenders gradually found that co-operative behaviour was rewarded. Catching children being good was one of the basic means of behaviour modification along with modelling and reminder time built into the week, and by means of songs and rhymes, etc. A good story time was concluded with praise and expressions of gratitude from the teacher for co-operation, contribution and effort. Children were also encouraged to thank their neighbours on either side for being co-operative during story time.