Just buy a new case and keep it in reserve: issue solved, pending actual failure.
Order up a new case, keep it, and drive your car.
Honda confirmed nothing, there isn't a lot of information, and problems were taken care of by Honda as they occurred. You are looking for something that doesn't exist. The transmission casing was cast by YSK in Japan, so I recommend you take up your quest with them, and good luck with that. I suspect Honda doesn't know and had YSK build and warranty the transmissions.
The snap ring fits tightly even on a bad case. At one of the group meets a depth gauge had been made, several cars were examined, and there were no discernible differences on any car that was "tested". I watched the whole process and decided it was too imprecise as the critical measurement cannot be made from the outside inspection port/edge of the case.
I suspect you are unlikely to determine with any confidence if your existing casing is in or out of spec. Even if you install a new case you probably still can't tell (though I am pretty sure new casings have some number that are black ink stamped on the exterior.
FWIW:
I did your same research when I bought my 1991 in the snap ring range (which failed at ~120,000 miles) some twenty-five years ago. At that time the problem cases were deemed ~1:25 and apparently now ~1:8. I suspect the 1:8 and 1:25 are rather pessimistic as half the NSX production was in 1991/1992 and yet there only appears to be a relative few transmission problems overall.
The car I purchased supposedly had the snap ring repaired and had a genuine independent shop invoice to prove it, but all the shop did was replace the snap ring. It was a big bill too, but wasn't well itemized. It failed and it's obvious because the gear shifter moves back and forth quite a bit. I installed a new casing+bearings+syncros+clutch, and that was that. Not a big deal because I could get the parts.
FYI: When I sold my NSX I handed over records and the old casing with the broken snap ring as proof to the new owner. The new owner was not a car guy and had no idea what I had given him. When he put the car up for sale, all the potential buyers only wanted to discuss the snap ring issue and how to verify it was replaced. It's a big deal. Which is why I recommend you get the new casing engraved at a trophy shop with the invoice number and date of purchase. Few will believe anything else. And if the casing ever goes NLA I can see extreme price reductions made for cars with "risky" transmissions.