Oliver wasn't a good leader, bless him, so he did need to be fired.
But I think Paul was a lot of trouble too. He was aggressive and negative and didn't sell. Also, I'm pretty sure he was the one who told Oliver to pick salt and vinegar fudge, saying it was fashionable, despite Oliver saying it had an unpleasant aftertaste. After demanding the salt and vinegar fudge, Paul failed to sell any of them! He should have got in more trouble for that. I think he should have been fired too.
I think Mukai actually did well. He made a mistake by down selling the extra sweets, but at least he had the idea to sell them extra. If he'd sold them at full price it wouldn't have won them the task. Therefore he wasn't responsible for its failure.