Since the employee is being fired (deprived of something), they are "the accused" and "the burden of proof lies with the accuser". All the charges that are presented must be presented in such a way as to be challenged. Allowing the manager the last word -- and not allowing the employee to even be aware of what was said -- indicates that accusations are being made without being challenged.
The selection of jurors -- three non-managers or one manager -- seems strange in part because it recognizes a hierarchy of people and in part because it involves such a small number of people. The validity of a jury hinges in part on a certain notion of the equality of people -- we don't have juries of 12 ordinary people or 2 senators in the United States because we don't expect that senators are "more just" than ordinary people. It's hard to see why expecting managers to be "more just" is any more reasonable.