To my knowledge, the Nashville Zoo has imported six small carnivore species that were previously absent from North American collections accredited by the AZA. I am not pretending to know what the reasons are for acquiring these animals, but I think three of the additions are easily justifiable.
The binturongs are subspecies whitei unlike the generic animals that make up a majority of the North American captive population. The Memphis Zoo I believe it was also participated in the original import so it is not like the Nashville Zoo's efforts with Palawan binturongs are completely unsupported. When it is possible like with binturongs, maintaining "pure" populations should definitely be prioritized over generic ones.
The Bronx Zoo has repeatedly demonstrated a commitment to ring-tailed vontsiras, and was holding them long before the Nashville Zoo was. Obviously two zoos cannot sustainably maintain a population of any mammal species long-term (with a few exceptions, of course) by themselves but with a second institution's support like the Bronx Zoo has now, it is a whole lot easier to kickstart a new population.
The banded palm civets have been breeding fairly regularly in Nashville and many of the offspring have been distributed to other zoos. This species was all but gone from North American collections prior to the Nashville Zoo's acquisition of a breeding pair, and now there are a few major zoos that hold an individual.
The ferret-badgers, Javan yellow-throated martens, and spotted fanalokas are more perplexing additions, but I wouldn’t say that there is any issue at all with the Nashville Zoo having them and breeding them. We actually don't even know where the ferret-badgers came, do we? Correct me if I'm wrong. I wouldn’t be at all surprised if these two came from the Flying Fox Conservation Fund in Chicago. That organization originally received their ferret-badgers from somewhere in Europe but they aren't listed on their most recent USDA inspection report, which is from last April. It seems possible that the Nashville and Chicago animals are the same pair.
I completely agree with you when it comes to animal ambassadors. Their purpose, to connect people with wildlife on a deeper level so that people might care more, is important, but personally, I think they are usually ineffective. The connection with the animal ambassador will be a memory cherished to some degree, but most often not inspire people to care more and do more, something that is honestly so much more difficult to achieve than facilitating a 30 minute connection with an animal. And as you said, how they are handled generally sends the wrong message about zoos in several different ways.
Well, to be fair, of the zoo's two most recent posts that have also been linked on this thread, only in the second are there pictures of a newborn animal separated from her mother and with blankets and stuffed animals. It is my understanding that most if not all clouded leopards born at AZA zoos in recent times were or are hand-raised. I believe this is because hand-raising cubs is considered the most effective measure to prevent them when they are grown from killing their prospective mates during intended breeding interactions.
I generally agree that the Nashville Zoo would be overall more impactful if they held species whose captive populations are struggling (unless of course their unique species end up just as common in zoos as other species, which is admittedly very unlikely). Where I disagree is with two of the species you mentioned as being examples of species that could be taken on instead.
Asian small-clawed otters are considerably more expensive than other small mammals. The costs of maintaining a suitable enclosure for otters, whether it is publicly accessible or not, is significantly higher than it is for other many mammals because they are aquatic and thus need pools and filtration systems. Their diets are also more expensive.
I've been told in the past that there aren't enough ringtails in breeding situations for the population to get much bigger. If that is true, then the problems the population is having would probably be best resolved by the zoos currently holding ringtails opting to manage breeding pairs instead of single or related individuals.
You are much more informed than I am, so please correct me if I am mistaken.