Clearly, the baking powder has had no affect, as the pH has dropped again, so I would suggest doing a 100% water change to get rid of it all, and then start again, increasing the pH with baking soda to 8.2, and then redose ammonia to 2ppm.
THe nitrate itself won't kill off the bacteria, the danger is that it converts itself to nitric acid, which obviously drops the pH again, and stalls the cycle, ie it inhibits the bacteria from doing what they do, it doesn't kill them. The good thing with using sodium bicarbonate is that it increases the carbonate hardness of the water (KH), which helps to keep the pH stable.
THe nitrate itself won't kill off the bacteria, the danger is that it converts itself to nitric acid, which obviously drops the pH again, and stalls the cycle, ie it inhibits the bacteria from doing what they do, it doesn't kill them. The good thing with using sodium bicarbonate is that it increases the carbonate hardness of the water (KH), which helps to keep the pH stable.