Sounds like you are already overthinking it...if you are part of a sting why would they go through the hassle of spending an entire session getting evidence...if you get caught in the LE web you'll be arrested once the money is produced.  I know that the rules have been extended to what LE can do to ensure there is sufficient evidence but I doubt they would work with a girl, have her complete the act, and then arrest you on the way out.  Sounds to me like LE would themselves be promoting prostitution

you do the deed.  Film at 11.