I have a couple of thoughts about this for you:
Thought #1:
Your KH is extremely low and there are hints that the N-bacs don't like that. You say that your pH is 8.0 (which happens to be ideal for growing N-bacs I believe) but then you say that its been dropping during fishless cycling (would be interesting to know, dropping by how much? at about pH=6.2 you can forget about N-bacs developing and down at 5.5 the bacteria will even die off.)
You'll notice that Lioness mentioned putting -more- baking soda in her tank. Baking soda brings the KH up and the pH up. If your pH is dropping way below that 8.0, then you could safely add baking soda and bring both parameters up. If the pH is sticking up at 8.0, then less baking soda would be in order. The way to think about an initial amount of baking soda is to use the following starting point: one teaspoon of baking soda (careful that its not baking powder!) per 50 litres of tank water should raise KH about 4 german degrees (your KH must be between 1 and 3 degrees since 17.9mg/L equals one german degree) without much effect on the pH. (to repeat: add 1 teaspoon for every 50 litres of water to raise KH 4 degrees without starting to effect pH very much.)
Now the other thing happening to you is that you are in the final stages but have not been performing large water changes. Large water changes, complete with gravel cleanings can have a really good effect on this last stage. It clears out the excess nitrites and nitrates and there is some speculation that these can slow N-bac development. By removing plant debris it also lowers an extra source of ammonia that may silently be pushing your ammonia beyond the 4-5ppm that you think you have (noting that at 8ppm a different species developes and slows things down.)
(I don't really think the plant debris would push it that far though!)
After you do a large water change is the best time to add your baking soda. You do your conditioner and add both the baking soda and the ammonia and don't bother testing the water until the next morning or so, depending on when you do the water change. If you have the patience to do this, choosing an operation like this and giving it a week might just get you down to 12 hours.
Thought #2:
As you were already thinking of doing above, keeping your initial stocking level to a medium size rather than fully stocking should prove that the filter is already qualified. Note that even RDD, who wrote our fishless cycling article has said this. Once your nitrites are dropping all the way to zero in less than 24 hours, but just not as fast as 12 hours, the filter is probably ready for a lighter load of fish. That being said, there are two problems you face: if the filter proves to be weaker than you thought then you will find yourself in a fish-in cycle, having to make repeated water changes. Also, the problem that everyone faces, of which fish to start with, is made a little more urgent since you know nitrite levels may not prove optimal right away - so you might feel confident about the zebra danios, but not about the corries for instance. And its a dilemma, wanting the fish load to not be too small, causing you to lose bacteria, but not wanting to expose more expensive fish like angels to a riskier early environment. So if you decided on this path, you might try to consider whether there is some other shoal of fish that is hardy to put in in addition to the zebras to help the fish load be hardy but a bit bigger overall.
OK, that's probably more than you wanted to read (but I wanted to get my mind off work
)
~~waterdrop~~