The battle of Saturn was Lopez' show, not Steeles. Sending out two Deimos Corvettes and an Aeolus to soften up the Wargods' fighter screen is a debatable, but not exactly stupid move. Also note that usually, both Corvettes survive the run, as does most of the Carthage's escort fleet unless the player explicitly moves to kill them.
Second, if Steele had doubts about Lopez' commitment to his command, he would have her replaced, NOT sacrificed like that. Steele knows that Lopez is far more valuable to the GTVA alive than dead. Could the situation have been resolved differently? Yes, of course. The Imperieuse could have been ordered into action as soon as the second pair of frigates showed up, but this is where Steele's psywar tendencies come through. He wanted to use this action to deliver the most crushing blow possible against UEF morale, which meant waiting for just the moment when the UEF believes to have won the day.