It is already discussed in the forums that the mercator seems to be overpowered. In the last game we decided to go with the suggestion that he only should be allowed to spend 6 money / activation. Still this was enough to get the last VP’s from my girlfriend close to game start as I was lucky to have him in the starting setup.I ended up just removing the Mercator card from the game and have been happy about it ever since
I've always played so that you have to decide which disc(s) to block at the moment you play the Praetorianus. Not sure where you got the info that you can wait to decide until after the opponent's roll, but don't think this is correct.
We were left with one question regarding the Praetorianus. We played it that you can decide which disc you block after the roll of the opponent. I feel that this might be a bit to powerful and deciding with activation might be the better solution.