Generally I think that
Reading between the lines and behind words bears a certain danger of reading 'into'
Tolkien's writings, especially when one has a certain tendency to let ones fantasy go wild...
The question discussed here, though, appears to me more like one simply looking for the primary meaning of a paragraph, than one looking for a veiled message 'between the lines and behind the words'.
Regarding sharpness: Maybe 'diligence' does the trick? Or exactness (maybe exactingness or exactitude too)...
cf. OED
sharp: distinct in outline or detail
If we try replacing sharpness with 'diligence' the thing makes sense...
(to me, that is)
Personally, I think one can't get much closer than Arv (in his first post).
The whole paragraph is part of an addendum/edition made about the time LotR was written, and the
field of Arda - quite curiously - read originally
field of the sun (cf. MR p.12), which - to me - explains the cone-metapher a little clearer. The comparison with a cone also makes the connex to another meaning of 'sharp':
'tapering to a point or edge' and here Tolkien's painting of
Taniquetil comes to my mind...
So - to me - the paragraph simply reads: Even though the
Ainur are so powerful, that their job in making/shaping Arda might appear quite simple, one should not underestimate their task, because they did it with diligence and in minute detail...