Disclaimer: I have not played SWSE and do not know the specifics of the adventure. I will try to keep this answer as universally applicable as possible. In my eyes, you have essentially four ways to go from here:
1) Willing suspension of disbelief
Basically, ask your players to use the same amount of suspension of disbelief they would use in a video game. No matter how urgent and world-threatening the main quest is, there is always room for a few side quests. When you come back from said side quests, the main one is still(!) super urgent. I am mostly mentioning this for completeness sake, since it’s not very pretty and probably not what you are looking for.
Heal them – somehow. If my Google Fu is correct, there is an alternative to resting: medpacs.
First Aid (requires a medpac): As a full-round action, you can
administer first aid to an unconscious or wounded creature. If you
succeed on a DC 15 Treat Injury check, the creature regains a number
of hit points equal to its character level, +1 for every point by
which your check result exceeds the DC. Using a medical kit grants a
+2 equipment bonus on your skill check. If the skill check succeeds, the tended creature cannot benefit from additional first aid for 24
Now, they can only be used once per day, so if your PCs already did that or if they don’t have the First Aid skill, a medpac won’t help. So reskin it – stims/drugs, maybe? A sudden helpful NPC with latent force powers in whose hand a medpac suddenly works way better than it should?
As the name says, this is kinda Deus Ex Machina and also not very pretty.
3) The PCs have to delay the consequences
Now we are getting somewhere. The PCs know they are in no condition to fight, but time is running out. So they have to buy themselves more time. (This is where I have to be really vague because I don’t know the adventure.)
Basically, keep the tension high by pointing to the ticking clock, but give them a (non-combat) puzzle/challenge the will stop the clock from moving for a while. Sabotage the shuttle, so it can’t leave. Block doors, so the Empire can’t get through. Hack into the computer system and cause mayhem and confusion. Ideally, this time should be bought the hard way and they should feel like they are laying the tracks in front of a moving train. Just stalling until they are ready to fight again.
4) “This is where heroes and cowards part ways.”
This is my personal favorite, to be honest, but your group might not share my opinion – this kind of DMing is very much love or hate. You be the judge of it.
Keep the urgency and do not give them a way out. If they rest now, bad things will happen. Maybe not all will be lost, but the plot will move forward, antagonists will act, all that jazz. Yes, they are ill prepared and in no condition to fight. The universe doesn’t care.
After all – what kind of heroes would they be if they only fought when they were sure to win? Give them a blaze of glory. Make it a truly desperate stand worthy of a movie climax. Maybe give them something that will make them fight beyond what they were normally capable of. (See the stims/drugs from idea 2#. Extra HP now – addiction later?)
If they survive – awesome!!! The hard-won victories are always the best. If not – that’s also a part of the game. And an epic death can accomplish the goal, too. (Rogue One!) When they make new characters, be sure to include the sheer badassery and the sacrifice of the original PCs in the ongoing story.