OK. Now for my humble opinion.
It doesn't matter that your vehicle was inpected by a mechanic at an Acura dealer which resulted in you getting information that turned out to be incorrect. People are human, machines are complicated and the world isn't perfect. Things break every day.
When I bought by 1991 NSX, I have the vehicle inspected by an NSX master technician at the largest volume Acura service dept. in the country (also a precision team member). The sales dept. at thie dealership is in the top 6 for NSX sales volume. If anyone should know NSXs, it's this place. They signed off and gave the car a raving review. BUT! If the thing had flat out died 10 minutes after I purchased it from the seller... it's my problem.
Anyway... back to your problem. You have a transmission issue, and you'd like to know what the chances are that Acura will fix it for free. From what I've read from you, and what I know of Acura and other manufacturers, your chances are not good.
You're asking for Acura to "goodwill" the parts and/or service to repair your vehicle. You suspect that the problem may be related to a snap-ring failure, as your transmission number is in the range of those installed into 1991-1992 vehicles which are suspected to be bad. You have also read that others have had Acura goodwill them the parts and/or labor to repair their transmission, even long after the manufacturer warranty expired. Thus, you are hoping that this will happen for you. Well... you need to also read about people who have been handed a sizeable bill for transmission repairs and realize that this may happen to you.
What determines whether or not Acura (or any other manufacturer) will goodwill parts/labor? Well... a lot of things. Most importantly, it has to do with the nature of the problem, the company's policy toward a known problem, and the relationship you have with the company.
Some manufacturers are REALLY good about things that they admit are their fault. Porsche is a good example. Go drop $100k on a new 911 and have the engine quit on you after the warranty expires. If it is something that Porsche knows to be a problem, they'll do the right thing. Boxster owners were experiencing engine failures, and Porsche was replacing the entire engine at no cost. I recently bought a MY2000 Carrera Cabriolet, and I'm VERY comfortable with Porsche's commitment to the customer. I also have a good feeling about Honda/Acura, under certain circumstances.
First, understand that Acura does a hell of a lot more volume than Porsche, and often times to "faceless" customers who buy Accords, Integras, TLs, etc. These are commodity vehicles. Customers who buy the flagship vehicles (like the 3.5RL) can expect a little better level of service. This is just plain reality. Life ain't fair. Customers who buy the NSX get even better "extra special" treatment. These are important customers to Acura... when they are an Acura customer.
If you bought your vehicle from the manufacturer, you get more points than someone who bought it used. Moreover, if you demonstrate "brand loyalty", you also get more points. The guy who buys an Integra, then an RL 4 years later, then an NSX a couple of years later from the same dealership and has all his service done at this dealership gets big points on the "customer relationship" factor of whether or not parts/labor get goodwilled.
I bought my 3.5RL new from Acura in the northeast, and had it serviced by Acura at the recommended service intervals. When I moved to Texas, I changed service centers, but still work with Acura even for oil changes. I had a window regulator fail on the passenger-side front door, still in warranty. Acura repaired it for free and goodwilled the parts/labor for the other three windows as a preventative measure even though they weren't broken. At $600-$700 for EACH window, this wasn't a cheap thing. But, I'm a good customer and they want to keep me happy.
I recently bought a 1991 NSX with 22,000 miles on it from a third party. The transmission is in the snap-ring failure range, but the condition of the car was immaculate (garage-queen), and the price was right. I very will may have a 6 speed transmission installed in the future anyway, so the snap-ring failure thing wasn't a big issue for me. However, I discussed this with the dealership when they inspected the car. Would Acura goodwill the parts/labor for me if the thing failed next week? Pretty good chance. Why?
Large volume dealer with whom I have a fantastic relationship asking for it to be done, combined with a long-term Honda/Acura customer (three Civics, one RL and now the NSX) and a low-miles car that is driven by a responsible person who takes care of his vehicles religiously. No, I'm not the original owner, but I had the dealer do a search for new OR used NSXs for me a full month before I found this one. If they had a new one, I would have bought it, but they didn't. This was the best combination of car/price/condition that I found, and made the purchase decision after discussing it thoroughly with this dealer. In fact, they wanted to buy the car for me, certify it and then sell it to me as a "certified Acura pre-owned" vehicle, but the seller didn't understand what was going to happen, so I ended up just writing them a check myself.
I don't want to discourage you in your efforts to work with the dealership in getting your problem resolved to everyone's (you and the dealer's) satisfaction. You just need to understand that the world isn't perfect, life ain't fair, and we're talking about a 8-9 year old used car.
I also hope that this rather lengthy post will be read by more than a couple of people and help explain the "goodwill" thing to those who don't quite understand it.
EDR