You say "Khufu's sarcophagus may have been the first granite sarcophagus, but the walls of the King's Chamber prove that the masons could achieve flat surfaces, as do the internal surfaces of the sarcophagus itself, so the concavities of the external surfaces were certainly intended especially as it would have been possible to finish off as flat planes if that was the specification. "

So far the earliest granite sarcophagus is to be found in mastaba17, and it would be nice to see a study on that.
I have sawn many things in my lifetime and created concave cuts, l soon learn to cut well to the outside of the line to allow for such things. The chamber walls are not created in the same way as the sawn outer faces of the sarcophagus. I should imagine that the chamber walls when constructed were laid with a sizeable stock complete with lifting bosses on their faces, indeed Petrie notes 15-16 remains of bosses in the chamber. These walls would have been dressed down, and a good example of this dressing process can be found inside the chambers of Shepseska's mastaba.
Likewise the inside of the coffer was created using a different process, and indeed Petrie reports an errant tube drill hole inside the sarcophagus. These two different processes can explain why we don't see concavity in these areas. The concave sides presented on the other sides from the master side are small errors, and visitors today would be unlikely to spot them with the naked eye, and neither do these errors effect the function of the sarcophagus, it is still functional. Extra care is taken were needed, such as the top surface were the lid slides on.

Nor can we take for granted that the mean measures are the intended true measures for the piece; the tolerances are quite large. This structure for over a hundred odd years has been bludgeon to death by mathematicians and pyramidologists. For example some in pyramidology see in the antechamber an expression of a square of 5 cubits and its circular counterpart with the same area, whose diameter would be 116.39 inches (based on cubit of 20.63); the square side being 103.15. Now the antechamber is spanned by 3 roof stones, so lets divide the square and diameter by three.
We obtain 38.80 and 34.38, straight away we have the external sarcophagus width and the internal depth, both being well within measured tolerance. I can develop this further to create the most amazing sarcophagus, but it doesn't make it true.

This is why l hate math, one can make no end of permutations; and in such a poorly built piece as this it can accomodate every theory.