The volunteerism is a way for the children to begin to explore the options that are out there for a placement, and even more so, a way for the leaders of the community to see where the children's aptitudes lie. If a child seems to do well caring for the elderly, then they will probably be given the placement of caregiver.
As for the second part of the question, I would say that is was believable that people would live in the community and remain ignorant of what release really meant. Jonas pictures it as a sort of quiet, happy occasion where the person just "goes away" to "elsewhere." People just accept that release happens, and even the people who perform the release ceremony believe that they are just doing what is necessary and normal practice. After all, it is upsetting and shocking when someone dies before their "time" -- such as when young Caleb died. The people in Jonas' community are accepting of release and are not shocked by it, because it is the norm, and it appears to be gentle and pain-free.
Starting at age 8, the children are "allowed" to begin volunteering to help out with various careers and jobs around the society. To call it completely altruistic volunteering is a bit of a mistake. The children are required to volunteer, and their hours spent volunteering are tracked. A punishment does exist for anybody who doesn't complete the set number of volunteering hours before turning twelve.
Once, long ago, it was whispered among the children, an Eleven had arrived at the Ceremony of Twelve only to hear a public announcement that he had not completed the required number of volunteer hours and would not, therefore, be given his Assignment. He had been permitted an additional month in which to complete the hours, and then given his Assignment privately, with no applause, no celebration: a disgrace that had clouded his entire future.
The purpose behind "allowing" children to volunteer in different jobs is for each child to begin figuring out what kind of career path he/she would like to follow. Children usually begin volunteering first for Recreation Duty. They help the younger children learn how to play and interact. Inevitably, a child ends up spending more and more of his/her volunteering hours at a particular job that he/she likes a lot. Each child's volunteer hours are carefully checked and tabulated at the Hall of Open Records. When a child turns twelve, he/she is then given a career. It is almost always in the field where that child spent most of his/her volunteering hours. In a way, by volunteering a lot in a particular area, a child can basically guarantee what his/her future career path will be.
As for the second part of the question, I do find it believable that there are individual people in the community who have no idea what happens when a person is released. In any given society there will always be people young enough not to have learned exactly what each and every phrase means. That's true in Jonas's society, too. The big difference is that in Jonas's society there are plenty of adults who don't know that "releasing" is euthanasia. The main reason for that is that there are only a few people who are actually involved with the lethal injection, and they don't talk about it. Another reason that people don't question what release actually means is that the term is a gentle one. Jonas tells readers that being released is sometimes sad, sometimes a thing to celebrate, and sometimes a dishonorable thing; however, in all of those cases "releasing" sends the message that the person is simply leaving the community to go somewhere else. The people think that a person is going away instead of being killed. Additionally, the Sameness keeps anybody who does know the truth about releasing from feeling strongly about it either way. They don't feel anything about releasing, so there's no reason the truth would make them stop wanting to be a member of the community.