Just because the characters may never find out who did it, doesn't mean the audience shouldn't get to. I agree with Fritzoid, I think it would be bad TV if the show never reveals to us, the viewers, who did it. But, it does seem plausible (and increasingly likely) that the characters IN the show never actually get to…

If he doesn't go back to that shelter and adopt that cat by the end of the show, I'm going to be so upset.

Doubtful. He probably barely even knows what Naz is accused of, much less who's defending him. He probably sees dozens of cabs being combed for physical evidence every week.

Fee, rent, whatever, I assumed it was a euphemism for quid pro quo sex.

Or traction for running away.

Yeah, she specifically said something like "You don't have to pay my fee anymore if you don't want to," which seems to imply that the sex was payment for the smuggling, and now that he's not getting his end of the deal, the sex can stop if he's so inclined. At least, that's how I read it.