Interesting problem. If your measurements are true then your AOB cycle is complete and your NOB cycle is stalled or completely failed and your Nitrite levels are currently stable due to plant actions rather than a NOB population. Even harder to understand is that you indicate you did see a nitrite surge then rapid conversion to nitrate, indicating an effective NOB population earlier. That is hard to understand unless you possibly did a water change without neutralizing chloramine and your AOB tolerance to cholamine was higher than the NOB, but that would a highly unusual scenario and since I assume you have continued adding Stability even more unlikely. (AOB = ammonia oxidizing bacteria, NOB = nitrite...)
Couple of observations: I am unaware of AOB ever inhibiting NOB and your biofilter quantity is sufficient or you would not have developed an effective AOB population, although I am curious about your setup, sump? if so what is your flow volume?
What I would do would be first; verify your nitrites. I am an admitted broken record on this but hate judging colors, hanna does make a freshwater nitrite checker, a little pricey but not compared to 6 Kenny discus. Alternately test some distilled water with your current test to at least verify 0. Also is your test measuring nitrite or nitrite nitrogen and does your nitrite vary with your ammonia dosage since you have gone from 4 to 2 ppm daily.
If your nitrite value is true then I would try a different source of nitrifying bacteria, e.g. API or Tetra product from LFS if fresh or Chewy. Frankly given that you have fish on the way I might dump both in the tank and at that point go back to the 4 ppm ammonia dosage. As a final thought your nitrifying bacteria use CO2 as their carbon source but can also use organic reduced carbon. I would add a very small dose of high protein flake food daily to promote growth.