I've also done some self-discharge tests in the last months (mostly on AA batteries though). From your tests so far, the unused Tenergy battery still has a higher voltage now after 6 months than the used one had after 80 days. If the unused battery keeps dropping at the same rate it did in the last 45 days, it will take another 7 months to reach 8,91v, at which point it will have self-discharged to the level the used battery reached after 2 months. We don't know how many mAh this corresponds to /we probably won't find out until the first used battery is depleted), but if the self-discharge takes 13 months to get to the same point as the regular discharge in 2 months (chances are it will actually slow down further), this would mean that the self-discharge is about 15% of it and the regular discharge combined. How much the actual self-discharge level will be is yet to be determined... I'd propose that if the used batteries get depleted, the unused one should continue to sit until it reaches 8,91 V on its own so we at least know how long this takes.

Your batteries are already holding up for a unusual long time (50% of the runtime of an alkaline although they have maybe 35-40% of its capacity), and it doesn't seem like they reach their end yet.