ceiling color and edge shadow

(picture attached)

Can anyone give a few pointers to fixing the discoloring of the ceiling(which is same material as walls) and get rid of the dark shadows at the edges?

Lightmass resolution of ceiling geometry is set all the way up to 1024(in desperation)

my lightmass settings:
Num indirect lighting bounces: 50
indirect lighting quality: 10

Ambient occlusion: on(with standard settings)