1. Referring to the small fleet? They didn't take out the defenses, the later Brute led fleet did. They could have used a microslipspace jump... but we can't see shit from the Cairo anyway. 2. 50 Gigatons is overkill, it could be 1 Gigaton but it wouldn't make a difference if the rounds missed :P. 3. Cortana specifically stated that the nuke would not stop the Composer unless the Internal Shield was taken down. This small mining charge was an excavation grade 30MT Havoc nuke.