Not that we would need to believe in an Emperor during the specified 300 years, not in canon terms. But we do know the Klingon form of governing evolved somewhat during that time, as a woman could not serve in the Council during Azetbuhr's time, yet Gowron was offering a Council seat to K'Ehleyr a bit less than a century later, and didn't appear to be sarcastic in his choice of bribe. Probably the form of government is what the governors make it to be, as long as certain ancient customs are observed - and those customs might be more ancient than the form of government, and largely irrelevant to the day-to-day practices of governing.
I think you're wrong about Azetbuhr - she was, after all, chancellor and no one seemed to object even though it could have been used to further undermine the peace initiative. It'd be very odd if she could lead the high council but not be on it. It was Lursa and B'etor who weren't allowed to take their brother's place on the council. Maybe the issue is something to do with Klingon noble inheritance/succession law rather than being a strict no-women clause, although clearly the result seems to be an all-male council.