This doesn't have to be an inconsistency. After all, what one ruler proclaims can be revoked by another.
Once Gowron becomes Chancellor, he can say that this law is no longer valid and put K'Ehleyr on the Council.
Or it could be that, despite what the novels said, the ban was put in place sometime in the months between those two episodes. Either way, the point is that Timo
was incorrect to assume that women weren't allowed on the Council in Azetbur's time. Both in reality and canonically in-universe, there's no evidence that the ban goes that far back. There was one novel, Sarek
by A.C. Crispin, that portrayed traditionalist, sexist Klingons reacting as though Azetbur's chancellorship was shocking, but that's all (and I don't think the Lost Era
novels are compatible with that one).
I just looked on some passages of Sarek and it is a bit contradictory. I did not read it, but there are ways to make it confirm: Azetbur could very well have been the first and only female Chancellor; the Klingon viewpoint can be subjective, some houses can be more progressive than others; well you get my meaning...
But thats neither here nor there.