OK, the fact that the key will not release it and the electrical does not move means the latch assembly is faulty.
#2 is the key cylinder part, 3 is the manual release lever and 1 is the latch assembly itself. If you can get the trunk open then I believe you can replace this yourself. If you cannot get it open then you may need to talk to a shop.
To get the seat out, use the following instructions so you can climb in the trunk from the front.
Push in and up on the front of the rear seat cushion.Lift up and out on the seat cushion.Remove the rear seat cushion from the vehicle.