Lasagna won't cook properly in oven with no lower exposed element :(
First, purchase an oven thermometer so that you can verify your oven temp.  Ovens can easily be off by 50 degrees or more.  Lasagna is pretty forgiving.  So exact temperature and time is not all that important. My process is similar...cook covered, then uncover.  Pasta turning to mush might be a result of overcooked noodles before the bake, or the lasagna being too wet. It also sounds like you might be getting uneven heating.  Try rotating the pan halfway through the bake.