I suppose the sinking wafers could do it, but that's assuming that they aren't getting eaten, which is unlikely if you have shrimp and Cories.
I still suspect the test kit.
I don't have any experience with Aquasoil, but I've heard it will leach ammonia for several months. 7 Months seems like an awfully long time though.
Maybe it's a perfect storm of all the previously mentioned factors.
Honestly though, with 50% water changes every week, the fert input should never result in higher than around 40ppm, assuming the plants don't use any of it. If you graph out the function of the inputs with water changes, you get a line with diminishing returns, such that the resulting concentration will always stay just below 2x the weekly input.
Again, if you want to stop nitrate for a week and see if the levels decrease, I doubt that one week will hurt the plants much. There will still be some nitrogen compounds in the water, even if it's lower than optimal. I'd dose K via some other compound though. It's been my experience that potassium deficiency pinholes can appear fairly quickly under high light.
It's just hard for me to believe there's 100ppm of nitrate in a planted tank. Especially under such intense lighting. This is why I suspect the test kit.
BTW I tried adding pure KNO3 to a test sample, and it didn't register on my API test. I'm not sure what conclusion can be drawn from that, but it sure took me by surprize.