Modifying the Morph Command

On this Morphing Topic, then...

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?

It's not difficult. In Brave New World, we changed Morph from a timed ability to a toggled stance-type ability that makes her take extra damage as long as it's on. As Madsiur said, making it persist is a simple matter of removing the references to the timers.

As for damage, remember that Morph reduced incoming magic damage in vanilla, so there's already a contingency for Morph modifying damage taken. Look at C2/0D15.

Is it even Possible to give a Character the Undead Properties (i.e. Cure Damage, Posion=Regen, etc.) without Inflicting "Zombie"?

I believe the Relic Ring does this, so yes, it would seem to be possible.