I didn't say anything about the animations (haven't got far enough to see them yet), just that it isn't essential for a jaw to be open to produce fire. It may look better, it may match people's expectations (especially when 'breath' is used in connection to the dragon's fire in the game), but if you want to argue 'realism', it isn't necessary for the jaw to be open.
Though there is a brief summary of the story between DD and BD in the D2:ED manual, and much is revealed throughout BD, there is also a BD novella (included in the fansite kit
(17.7MB) with a summary here
) which would fill out the story in a little more detail.
Welcome to the forum.