Limit Breaks are just natural abbilities used by the characters. There is no link between them and sorcery. Also, battle based skills very rarley relate to storyline aside from a few exceptions, such as Terra's Morph in FFVI, Garnet suddenly getting a lot weaker for a short time in FFIX, and as you mentioned Angel Wing.
Also no one is Ultimecia, the only person I can think of her being is now dissproved (not an FFVIII charcter) bottom line - Ultimecia is Ultimecia.