A 'dragon', could just be a reference to a dark, evil creature. Dragons are typically not the most light-oriented creatures in mythology (it differs from culture to culture and story to story, of course). Born fron a dragon probably just symbolizes someone who started out/lived most of their lives as a sort of dark person. They are most definitly talking about Cecil, the game makes that abundantly clear time and time again.

As for Cain in the logo... well, you said yourself Yuna was in the logo for X (I dont remember offhand), but she isnt the main character, Tidus is. Cain is very very important to the storyline of IV.