I may be completely on the wrong track, so don't take my concerns too seriously. If you have real words at 12d and 18a but nowhere else you have found a way of resolving the clashes that I haven't seen. The grid contains a number of instances of a certain letter. Discounting the cells with clashes there are 21 or 22, depending on the choice for the ambiguous entry. There are another 6 instances in clashing cells. If they are involved surely all must be or none, because how would one choose between them.To me that means the answer must be 21, 22, 27 or 28.
Surely only the first can be arrived at by the equation of square root of a number + 8/9ths of the number plus two for the other two.
Perhaps I'm missing something. This is, after all, Sabre, who is the subtlest of them all.