a: I was looking at the Greek gods thing for a moment, gee a lot of people can be immature. In Greek (and Roman) mythology, the gods can morph into anything they darn well please, whether it's some animal or some human in a different form.
b: OK, originally, Ganondorf wanted to help his people from their brutal conditions (see his speech before reassembling the triforce in Wind waker). However, after researching the Triforce, he concluded that this can only be done by force. this essentially corrupted him. The triforce of power corrupted him even further (and may have led to his dehumanization too). Neither Zelda nor Link are power hungry (Zelda refuses to rule by iron fist, and Link prefers NOT to rule), so they aren't good candidates for the triforce of power.
c: Twilight has NO corruption powers WHATSOEVER. It just transforms some light creatures into lost spirits and others into twilight monsters (namely NPCs into spirits and monsters into twilight). Midna has expressed that Most twilight beings were peaceful and kind (learning the past mistake of their ancestors), and to represent shadow, Midna originally had a devious and semi-selfish nature (Note the constant taunts and original lack of care of the light world [she cared about her realm, just not Link's]). She became more serious and less selfish after Zelda sacrificed herself to heal her (Seeing the constant sacrifices of Link and Zelda, she decided to help the light world too). Link was protected by his triforce piece, end of story.
d: Zant was originally slated to be king, but he was upset that the Twilight were trapped into the twilight realm and that the Light creatures exist freely in their own realm. His anger and lust for power made everyone choose Midna over him (Midna being kinder and non-power hungry). Upset, Zant encountered Ganondorf, who gave him some of the power of the triforce, which helped him take over and further any Corruption of Zant.
e: In terms of Link's intelligence, Link is actually quite similar to Modern kids (I'll use Bart Simpson). Link has a high IQ, very capable of solving puzzles and find weaknesses in enemies (Bart can come up with elaborate schemes and has a high mental agility). Conversely, He isn't knowledgeable, often acting without thinking straight, if not thinking at all (Wind waker is full of occasions). Bart has a lack of knowledge too, not knowing what the Equator is.
