Unlocking Imperial Marches Retail level after the event ends?

I was in the same boat. (Playing under iOS on an iPhone 5, btw.) After making sure I had no pending tasks, I set my device date back by a week or so (i.e., closer to the start of the event). I did lose a bitizen out the airlock (worth the one-time sacrifice, I thought), but was then able to proceed by starting tasks on each level (usually the higher-medal tasks where available) and then advancing my device clock two hours — long enough for all of the tasks to complete — then repeating the process. Managed to unlock the droids (which all show up at once — I originally thought they were three separate rewards, but no) and Imperial Marches. (Note also that it appears that you only lose a bitizen when setting your clock backward, not forward. Kind of critical for this plan.)

Now I'm curious what will happen if I set my device back to during the decoration event — since I missed getting Panna City Medicines — but I suspect that since the app has been updated since, it probably won't work. Similarly, I'd suggest doing this before the Imperial Service event ends, just in case there's some kind of "flag" that goes off in the app once the event concludes that'll prevent you from going back after it ends.

Update: As I suspected, no luck on setting my device clock back to during the decoration event. Lost a bitizen out the airlock, but the device immediately triggered the end video for the service event, and no decoration event showed up. Fortunately, once I set my device back to the current date/time, the service event appeared to be back on. So you may have a few days (presumably until the next app update) to take advantage of the date-switch strategy.