[X]You would be swift to apologize and make promises to improve: you had a suspicion that the Council's complaints were meritful enough this was at least mostly justified, and apologizing and promising to do better worked in more contexts than being scolded by Maman.
[X]You would focus on 'just the facts': stay away from seeming like you're defending Caras or attacking the Council, and focus on providing the overdue reports. Perhaps the Council really only cared about this information?
[X]You would use cluelessness as a shield: ask questions, admit ignorance, and pretend as if any verbal barbs were unnoticed by you. (Given your experience in Soissons thus far, you might end up half-doing this without meaning to...)
[X]You would focus on the ways this wasn't Caras' fault: the newness of the Burner Worms themselves, the other Lords making things harder on him, the 'no soldiers' rule constraining him. (You'd certainly be very diplomatic about raising that last one!)
[X]You would try to emphasize that things were currently changing for the better: whatever their complaints were, you were confident that in five years' time they'd be handled.