It seems to me that Brittany Ferries are assisting you all they can but the loading crew seem to ignore requests from management. Unfortunately, with the Bretagne being the oldest of the fleet (1989), there's not much they can do regarding easier access. Some of these powered wheelchairs are rather large which makes it difficult to access the ship compared to a standard chair. Fingers crossed when the new ship comes into service and vessels moved around, you may get Pont-Aven to St Malo. Perhaps someone else could drive on board for you and you used the foot passenger route to board the ship?