The shank of the Torx-15 driver should be 8in or more.
It is an inescapable outcome of the ageing of light-coloured ABS that it visibly yellows. You can never do more than minimize the resulting colour-change.
de
By keeping the Mac in an oxygen-free atmosphere, out of direct sunlight and away from heat. That will minimize oxidation of the butadiene component of the ABS plastic. Of course, it will also minimize your use of the Mac, as you will become more than a little breathless while you are using the Mac in the oxygen-free atmosphere. So don't do it often. Better still, don't do it at all.
It is an inescapable outcome of the ageing of light-coloured ABS that it visibly yellows. You can never do more than minimize the resulting colour-change.
de