So, to further test them for long-term effects, I partially submerged one fitting in a mixture of 1:9 bleach to water. That should rapidly accelerate the rate of corrosion. (Don't do this.)
Needless to say, I am impressed by the results.
Almost all of the plating remained intact on the fitting. Minor corroded nickel sediments can be found around the collet. Some very minor nickel flaking can be found in the space between the collet and the main fitting body. The tiny black sediments can easily be wiped off.
This flaking/corrosion, better illustrated in this picture. This is actually outside where water would go and thus irrelevant for the purpose of this experiment - finding corrosion on the inside of the fitting. This was the most serious issue of degradation in 24 hours.
The inside remains identical. There were some marks on the inside prior to submerging to bleach. Possibly scratches on the surface from the copper pipe, but these marks did not grow significantly in size nor show any sign of oxidation upon close inspection. They just remained marks. None of the teeth displayed any signs of damage.
The threading and rest of the body are totally unscathed.
With that, I'm confident enough that these won't have a meltdown under water.