From what I understand Imperium referred to both military rule and legal rule. I'm guessing that, for the Aediles and Praetors this referred more to the legal component. But are there any instances of Aediles and/or Praetors leading armies when either of the Consuls weren't available?
Aediles didn't have imperium, that's why they didn't have lictors. Praetors frequently commanded armies (and Varro's proposed etymology for the term indicates the Roman conception that it was originally purely a military office), there are loads of examples, and propraetorian magistracies were a critical component of the post-Sullan provincial administration. Before the establishment of regular provincial promagistracies as an extension of the elected office of equivalent rank praetors and consuls were regularly sent to the provinces for administrative and military duties (really the same thing--to be governors) for the entirety of their year of office