There is another way into that part... I don't know exactly where as I got stuck in the same part myself and I stumbled upon it 'accidentally'. I think its through a collapsed wall in one of the tombs.
in one of the key rooms there is a second coffin on the right through the place where the skeletons have broken the wall also near that door you were talking about is a door that is stuck half way open roll under it to find another passageway