so which is more appropriate: iedere or elke? I used elke and it was fine but are these words interchangeable?
yes, these words are interchangeable in practice. But that's ok, so you can sometimes varify your sentences
I feel like when to use iedere vs ieder (and similar cases) hasn't been very well explained yet. Lost a point here because of that.
See the explanation here on when you need to add the -e and when you don't.