Thanks for the response, Raymond. To be clear, play balance is the last of my concerns, and no concern whatsoever in the historical scenarios. I flatter myself with the thought that they might add to one's understanding of (the mechanics of) a battle.
The French forces arriving near the end are indeed useless; they are D'Erlons Corps which had been marching between Quatre Bras and Ligny and Quatre Bras... hence the fatigue. By the way, the Allied side also receives reinforcements that arrive too late to play any meaningful role. These useless reinforcements serve to illustrate that Quatre Bras was "a meeting engagement in which both sides struggled to get the necessary forces on the battlefield" (to quote myself).
The scenario you played gave you the same restrictions Ney faced, notably in the use of his cavalry. Scenario 3.1 takes these restrictions away, so you might want to try that one; the set-up is the same. Or 3.2, which gives the French player more freedom to choose his point(s) of attack, or to start harassing De Perponcher early on. Or, of course, Scenario 3.4 if you still can't win
Hope you enjoy!