3rd case is beyond me. It isn't a simple formula. There are whole papers written on the subject.
If the weakest gravitational wave could rise the fluctuation of the LIGO interference fringe of light, how should we neglect the gravity’s influences on light?
If light is held by gravity, then measuring the speed of light anywhere on the earth will be a constant.
If classical physics can explain the Morley experiment, it can explain the bending of light, and it can deduce the mass energy equation. So is it necessary to assume that the speed of light is constant ?