It depends on whether the GM wants to handle the whole encounter as a single check, or make multiple checks. For a single check, use Deception - the PC needs to fool the Imperials into thinking she's a legitimate officer. For multiple checks, start with Deception to make sure the Imperials aren't suspicious, then Leadership to get them to do what she wants. Assuming she passed the Deception check, then I think obedience would be automatic (unless there's a Despair or a hatful of Threat on the Leadership roll, in which case she's made a mistake in the order she gave that could tip of the Imps that she's not who she says she is), but a good Leadership roll would get them to work faster, more diligently or give more than she asked for. There's a reason good leaders get better results out of their troops, those troops are willing to put more effort in for a good officer. Advantages or Threats on the Deception roll would add Boost or Setback to the Leadership roll, obviously.