Hmm....well, Nathe's objective is to learn about the forest elves' culture right? So, what if somehow during the interrogation he tricks or convinces them into allowing him to wander around semi-freely? (perhaps something to do with the level of his injury, maybe he spends most of his time in an infirmary of sorts while they treat it to make sure he doesn't die or something?) While the forest elves at the same time attempt to extract 'secret' information about the Drow colonies in the caves to give them the advantage in the conflict somehow? (Since I don't imagine the forest elves would have gone awfully far into said caves, probably being afraid of ambush or some such other thing)

We could probably have them put him under guard if they do allow him semi-free range, perhaps Mel volunteers for it under the premise that he's "more likely to trust her and give her information freely since she already helped him"? 

...I hope this makes sense and is more than just a nonsense brainfart XD