Doubts

2) detergents reduce angle of contact of water with the threads of the cloth so that they can easily pass through the pores of the thread...

considering the pore to be a capillary tube, the height to which the detergent-water mixture will rise will be more than the height to which ordinary water will be raised.........
thus it wuld help in cleaning away the dust particles from within the spores by allowing easy passage of water through it.....

h=2Tcos (a)rdg

a=angle of contact; r=radius of pore between the thread; d=density of liquid

if 'a' is decreased cos(a) would increase and hence the height will also increase...

4) gravitational field is independent of the presence or absence of a medium.........hence there will be no difference even if u place a body in the hollow sphere