I haven't played this game and I don't know about the mdec thing, but maybe you could use a software renderer to finish the hedge maze part. Its only a guess. A different GPU may also fix the Mdec problem.
Nope, using software rendering didn't help either. It was like that part in Chrono Cross inside this tower where a texture-thingy was overlapping the middle part of the tower, in both bleem and epsxe. As far as i remember, vgs didn't render it correctly either. Are those textures supposed to be translucent or something?