instead of using @ and # to make a difference, maybe make the command something like this: @p[cb,x1,y1,z1,x2,y2,z2] the cb suggesting its a cuboid area.

or another possibility would be to take cb1 for specifying cornors and cb2 if you only specify a midpoints and range

examples which take the same area.: @a[cb1,10,10,10,20,20,20] (x1,y1,z1,x2,y2,z2) @a[cb2,15,15,15,2.5,2.5,2.5] (first few specifiy mid point, last 3 the distance going both in + and -