Why does it not accept "Kessa, Daenerys." ? I thought 'kessa' meant 'yes' just like 'issa'?
It depends on where it refers to. "kessa" is used if the previous question's verb was in the future tense. eg.
Havon sinditā? - Issa. vs. Havon sindī?? Issa. vs. Havon sindīlā? Kessa.
At least that's how I understand it. So "issa" is the most general term for yes