bert I'll answer, but theoretically I'm not pro grammar except for super selectives. My stance with the alternative of the current comprehensive system is 'they aren't any more unfair'. And my answers apply to my local fully comprehensive area, where there are marked differences between schools. I accept that in other areas the answers may not be applicable.
Anyway
a) better for those well above cut off, however those closer to cut off would miss out to advantaged peers of equal or slightly less ability. So would see no change, they'd still get the worse school.
b) no different, they'd still get the worse school
c) no different, the less advantaged would still get the worse schools, except for the more advantaged who would be worse off as they would no longer be priority for the desireable school because of their postcode.
d) Same as c.
Basically you'd be swopping the advantaged middle and low achievers for the higher ability disadvantaged. So no overall difference statistically, but for the individuals who are better off possibly life changing.
I don't think it would be fair, as the all round advantaged able child would still get the best, and I don't think it's ok to offer a lesser education to anyone. But if it comes to a trade off between otherwise advantaged low and middle achievers, and disadvantaged able dc, the latter need the good education more.