Considering it's such an important event and triggers the ending of the game, I doubt you can change it.
It's really awful too -
There are so many missed opportunities for great C&C that simply get dumbed down to plot-required stupidity. This makes it insanely frustrating when playing through the game a second time, when you know all these points where the game should've let you act. If this game had proper C&C it'd be amazing, and it's a damn shame Bioware got lazy.