ad 5 - the two coffins: This is the entrance to the next lower level. The graves open, when you open the four around them (you found that out already)- Either of the middle ones is the entrance downstairs - irrespective which of the two you choose, it will bring you to the same point, and the other one will be automatically locked forever. No mystery there - no opening need or possibilty either.
ad 8 - In the middle of Ars Magickana marketplace is a possibility to go "underground". If you turn east then, soon you will hear the hiss of the singing blade <img src="/ubbthreads/images/graemlins/evilgrin1.gif" alt="" />
