My guess would be that experienced military men like Ewell, Cleburne and Lee had a good "barometer" of what was possible. But each one of them stressed that the crucial key was emancipation. If the armed slave had a strong stake in the outcome, would see himself and his wife and his children freed, they would have fought like tigers to achieve it.

If sent to battle with nothing better than a tepid dicey "maybe, we'll see" on emancipation, I think all bets were off. That is why Lee wanted nothing less than a clear unequivacable unconditional emancipation.

Lee wanted independence over slavery.