I'd guess the kid that got raped as a result might remember. The point is sound though, there's not likely to be any clear testimony on this topic and even if there was it would be easy to dismiss due to the time that's passed. I can't think how there'd ever be any hard evidence of such an assignment, either.
This isnt really the issue in terms of misfeasance. You don't need "hard" evidence - testimony is enough. If Candace testified that the elders assigned her to work in field service with her abuser, the jury would be entitled to believe it and return a verdict, no matter how long ago. The issue is that her lawyer is saying on appeal that they were assigned to work together, but he never directly asked her that question when she testified at trial.
That suggests to me that Candace couldnt testify to it - either because it didn't happen or she can't remember. If she could testify to it, and Rick Simons just didn't ask her when she was testifying, that would be a very bad job of lawyering - it's an extremely important piece of testimony.
The only other thing I can think of is that the lawyer didn't understand how JW field service works and he thought the papers on the information board actually said who the door-to-door partners were. That seems unlikely though - he seemed to otherwise have a good grasp of WT and congregation policies.