je ne l'ai entendu. Isn't pas optional in casual spoken French? Or did I do something else wrong here?
It's actually the opposite: the 'ne' is ommitted in casual spoken French
(Je ne l'ai pas entendu -> Je l'ai pas entendu), but this ommission is not standard and thus usually not accepted on duolingo.
Why we used entendue here instead of entendu
It depends. In this case, "it" in this sentence refers to a feminine word
What is the difference between ecouter and entendre?