Because Gabriel was not assigned a couple and failed to be able to sleep soundly in the nursery, he is going to be released. Being released in Jonas society is the same this as being executed by lethal injection. However, the citizens do not see release in this manner. Their society is a logical society and Gabriel does not fit in correctly.
A Murmur of Replacement Ceremony is performed when a child that has died such as one of the children who drowned has his named faded away. The name is later able to be reused for a new child. It is at this time the name is re-spoken until it becomes the new child's name.
Roberto is not replacing a lost child. When a child is released, or put to sleep, then he is no longer a part of the society. He was chosen to be released. However, a child that dies in an accident is a different situation.
Parenting in the community is different in that the parents must be ready to have a child. The three years give the parents time to become ready for a child and to ensure that they match and are well suited for one another.
FOr the first question you have to keep reading to find out. As for the second question, a new child was named the same name as a child who had drowned in the past. When he died the community said his name all day, softer and softer until it faded away. The child who was given his name was the replacement child so his name was then spoken softly and then louder and louder.
New child Roberto did not have a murmer of replacement because he was not a replacement. Old Roberto had been released so his name was "available" for them to use again.
The couples had to wait to be sure that everything would work out with the child that was given to the couples and that it was a good fit.