I worked in the automotive industry for over 10 years, and handled dozens, if not hundreds of safety recalls. It was never the practice at any of these service centers to do anything other than the recall. Of course, if the technician noticed another problem or service recommendation, it would be noted on the repair order, but we NEVER charged a diagnostic fee. The important distinction, here, is that those car owners got back the same vehicle. Nano owners will get another device.
I've also owned products that were recalled, and it was always handled at no cost to me. There's absolutely no point in going through the rigamarol of engaging the customer over discovered problems unless, as I said above, the unit has been severely altered (I used the absurd aquarium example for comic relief, but a more realistic example might be a Nano that was smashed under a car or mangled beyond recognition. Even in that case, I truly think the techicians who initially screen the units are either going to say, "whatever," and toss the thing into a bin to be destroyed, or simply return the device. It's bad press to engage in disputes with customers during a recall. Toyota learned its lesson over this very thing.
I also think that it would be bad press to not include a new case in the refurbishment. They are probably outsourcing the remanufacture of new cases to Asia, for pennies per unit. It's just plastic, and the retooling needed to mold them is child's play. It may still be in place in some boutique sub-contractor's factory. Also, rather than manufacture thousands ahead of time, as implied above, I'm sure they've arranged to have them made in lots, using the just-in-time manufacturing supply model, hence the up-to 6 week turnaround.
As for a unit not needing to be replaced, I would think that this would only apply to units that have had the battery replaced already, and I'm sure this can be determined very quickly, perhaps via a computer diagnostic without even removing the case. Then of course, the unit would just be sent back to the owner, but no one is going to get charged a penny for anything during this recall; I'm certain of it.