9 is ruled out by the rest of sentence because the factor 3 (one less than a multiple of 4) occurs an odd number of times in the factorisation.
The opening sentence of the preamble doesn't meant an entry or grid number coulddn't be associated with more than one entry in theory. There are 6 possible grid numbers satisfying a; there are two satisfying c. I'm assuming that the two that satisfy c are ruled out of consideration for a.
I may be wrong, of course. I'm pretty confused myself. I do have 11 tentative entries that fit together, though I have only three of which I'm pretty certain.