Your pH is dropping because there isnt enough carbonate hardness (KH). The microbes responsible for the cycle use KH in the process of turning ammonia into nitrite and nitrite into nitrate. The crushed coral is adding KH as it slowly disolves, but it is being used by the nitrogen cycle quicker than it is going into the water from the crushed coral. From your post #5 you said that the crushed coral wasnt working so you started to mix in baking soda with your water and that seemed to be working at keeping your pH higher. Presumably you stopped doing adding baking soda with your water changes.
The natural processes in aquariums tend to make water more acidic. KH in the water absorbs acid, so it will raise your pH, and prevent that natural acidification happening. Adding KH is called buffering as it creates a buffer against acid. Once KH is used up the buffering stops and those natural processes will acidify the water and your pH will start to drop. Thats what you are seeing. And as said, the microbes need KH to turn ammonia onto nitrite and nitrite into nitrate. So when KH gets depleted the nitrogen cycle stops functioning. This is what happened in your tank before you started to add the baking soda, and why your tank didnt cycle.
But also, as we said, ammonia isnt toxic in acidic water, so while the nitrogen cycle doesnt function at low pH, it doesnt matter. You dont need to cycle an aquarium with a pH as low as 6.
If you are no longer going to buffer the water you dont need to cycle the aquarium. You say your fish where happy and healthy. If you want a higher pH, you need to continue adding the baking soda to a level you are happy with, that will raise both the pH and KH and allow your tank to cycle properly.
Whats probably happening is that the crushed coral is adding a little KH, which is being used up cycling the tank, so you are seeing some nitrite, but it quickly gets used up which could prevent your aquarium cycling properly and is causing that pH to keep dropping.