That's what many people forget about. MC isn't a VIP, a Commander in Chief of the UNSC. He's just a soldier, he has to obey any order he might receive and he has not to argue with the commander (as he did, when Del Rio stated that sending a recon would slow them down). Yes, of course everyone should listen to his advice, i first would listen to him and obey, but in that particular case, Del Rio had the full command. And by the way, everyone thought he was dead, nobody expected him.
But he was put in Command of the Infinity by Lord Hood himself, if he was a bad commander, whose the fault? He just acted as a Commander should do. No proves of what MC said, just a rampant AI and a thought-dead old soldier. What should you do? Engage an unknown enemy with a relatively intact ship, in a forerunner world? With a Covenant fleet awaiting?