Modifying the Morph Command

How hard is it to make the Morph last for the Duration of the Battle, and, rather than Doubling Battle Power per the OP, making Terra receive, say, Double Damage from Magic Attacks, or even giving her the Undead Property?

For make morph last the whole battle, you just have to stop the counter of the gauge. I did something similar in a small hack a while back (see this). You could also with some modification to the code double the damage received from magical attacks. I don't think it's that hard, although it requires you to be able to do basic coding.