Keep in mind though, that even if Udina couldn't order the fleets of the Citadel races to earth, he could order the Citadel fleet and the Spectres to aid earth which is what I presume his goal was, to save more lives/give Shepard time to get more help. It's still a very desperate course of action and more then a little foolhardy, but I have to imagine Udina realized that. And once he was committed to that course of action...well...there was no turning back.