That said, I don't really see
why all ideologies have to be equally playable. This isn't like Starcraft - you don't pick an ideology to win. An ideology is a reflection of what happens. An absolute monarchy in 1900 usually happens either when your country is sitting on a powderkeg owing to political development not keeping up with social changes (Russia) or is hopelessly backward (like Turkey). The game isn't about ideologies competing (for that, see Hearts of Iron

), it's about countries. So whether all ideologies are equally playable (and, by playable, I assume you mean not just interesting, but also capable of dominating the game) is the wrong question to ask.
I think you are confusing the issue: absolute monarchy isn't an ideology, its a form of government. The fact is that there are successful examples of both Conservative, Liberal and Socialist governments within the Vic time-frame. And while I wouldn't call them successful, there are enduring examples of Communist and Fascist governments (Reactionaries as well during the early part of the game).
These ideologies where all clearly viable governments in that they could achieve and maintain power for long periods while advancing their programs. And this should be reflected in the game.
Moreover, if you forget about the extreme ideologies, and focus on the moderates, it isn't clear to me that any of those three systems was intrinsically superior for state building. Germany and the US both managed incredible modernizations in spite of being governed by different types of parties. In VIC1 however, you almost never wanted a Conservative government. LF Liberals where superior for developing your country early on, while State Capitalism socialists where a superior choice in the late game, which is why Japan (which was able to freely switch from one to the other if it got lucky with the Meiji event) was so powerful.
And lets not forget about immigration: strategies for any American nation generally began with "Switch to a Liberal party to attract immigrants and then...". I see what this is trying to model, people did escape European autocracies for the greater freedom of the US or Argentina. But it would be MUCH better if they came not because you have a Liberal party, but rather because you have Universal Suffrage, Public Meetings, and very lucrative job opportunities.
Making the game interesting requires that Liberals not be intrinsically better than Conservatives (or Socialists for that matter). But rather that the best choice be contingent on you country and on the situation.
But to come back to your example, was Russian autocracy destroyed because Reactionary Absolute Monarchy is intrinsically doomed to destruction? Or because it refused to grant political and social reforms, mis-handled economic modernization, and got mauled in an unpopular great war that it was unwilling to quit?