It stops working after she absorbs one spell, unless you continue to use it. So let's say I Runic for 4 turns, and then an enemy launches a magic attack. I will absorb it. If I use it again, it also allows me to absorb the next attack. But if it uses two magics, then I will only absorb the first. The MP absorbed is the amount used to cast the spell (so Ultima would absorb 80, while Fire would absorb 5). This essentially means Runic does not stack in amount of spells absorbed per usage. The sword equipped for Runic must be a "Runic" Sword (which is most of the swords in the game, so don't worry about it), and same thing for Gogo, Runic will not stack, so whoever uses Runic last will get the MP.

Morph also doubles the amount of Magic Points after the battle if Terra is still morphed when the battle ends. It boosts Defense, MDef, and Magic Attack (not sure about Physical damage). AND this cannot be Mimicked.