I have a wrongful dismissal case going into mediation in 5 days (after 19 months of being put off) the board 9f directors responsible for my termination are sending my replacement (executive director) to defend their position. This person was not there and isn't a board member. Is this appropriate

Generally at a mediation the merits of the case are not argued per se. Mediation is a confidential and out-of-court process to attempt to facilitate the settlement of a matter. The Corporation has to send someone to mediation who has authority to provide instructions / any offers to settle on behalf of the corporation. If the Executive Director has such authority he/she can attend.