I haven't gotten a chance to test it yet, but what about making the Frontier pile remove itself from the game after 8 rounds? It makes the availability the same for any number of players, removes the interaction with Graverobber and Rogue, and marginally decreases the first-player advantage by not having an odd number of the card half the time. Maybe even more importantly, it removes the "one pile down" condition by no longer being a supply pile - I just hope you didn't plan to use an Ambassador on it. Obviously I can see an issue with keeping track of how many rounds have gone on, but it preliminarily feels less swingy, if such a card could feel less swingy.
On the other hand, perhaps the first player having more Victory cards so early will benefit the second player in the long run.
I also toyed with the idea of removing one on every shuffle instead of every round. This way, you still keep them for about the same number of rounds, it scales with 3 players the same as 2, and there is an odd number of them less often. It might have some strange interactions with deck-cycling at that point, so that would need some thorough testing.
BTW, I'm varvarop2 on TTS, since my name can't start with a 4 on an IRC. I plan on commenting on the cards after testing them, but this one was curious enough to comment on first [and on things not yet mentioned.]
