My idea just right now, would be to reach and land the planet, and hide our starships: the assault transport in one place, and the X-Wings more carefully hidden somewhere else (I don't know, 2 or 3 miles away).
Then, we should try to find and steal discreetly an imperial starship; or at least a starship that is duly regisetered and authorized on this planet; yet a military imperial starship would be better. To do that I think that our shapechanger could do great. I could accompany with a uniform and make the Bluff or diplomacy checks if need be. We then hide this starship not too far from the others.
Then, we go into the sewers and try to do what we have been asked: recover the information, free the 3 officers, but also the others. Then go back into the sewers to the starships.
Once there, let the people freed take the assault transport; and let the X-Wings make a so-called diversion. We will "pursue" the X-Wings in our stolen imperail starship, while the assault transport do what they can to reach the Rebels Alliance base... If we have problems, we can say we are in charge of the situation, and hope the imperials let us pass (I could say they have to go and look at the assault transport, while we "pursue" the X-Wings.)