i think new stall speed is proportional to square root of the increase in weight ie.
Square Root (320000/285000) = 1.0596
So new 1.3Vs = 180*1.0596 = 190.73 kts
For question 3 i would suggest max takeoff weight will be restricted by performance limited landing mass which is 54000, trip fuel is 3000 so max takeoff weight is 57000.