They don't want the government's help. Mellisa is the government. That's why they'd refuse.
Though, they may change their minds after they realize who she is...at least in regards to her personal help.
What I'm actually expecting are a lot of defections from the next batch of attackers. You know, all those Batarians of 'quesionable' loyalty that are easily sacrificed by Command. ;)
Wrap quality guns in thick armor. Pretty sound strategy. Just need to make sure they can traverse enough to target quickly.
Answer: Ortillery. Might not be the best call as the objective is releasing 'living' slaves.