From my point of view, there is no issue about bacteriological hazard during a frozen shelf-life. Parasites are killed when frozen (that's why some fish species have to be frozen for 24 hours before to be used for sushis) and bacterias don't grow at such temperatures.
-18°C has be chosen just because it is 0°F, but any temperature below -10 or -15°C will have the same effect.
The evolution of frozen foods is linked to organoleptics issues. A frozen product can dry, and after some time, the defrosted products will not have same characteristics than a fresh one.
However, i don't know any possibility to make accelerated tests on that purpose. Maybe, if you have old samples, you can find some that are near the 18 months of storage.