Very Worried

Probably best to not add anything new

Leave it for a few weeks, dead animals n all
At this point

Let it cycle completely

Do a water change after it cycles
I agree with others to get another test kit if you haven't already, just so you can be more certain of accuracy. IME, API is very spotty (Their Nitrate test is even worse.) Red Sea also works very well as another option to Salifert. If you really do have ammonia and are still struggling to get it down, IO Bio Spira works quite well as a bacterial booster to counter it and help get things stabilized.