3.) Someone told me that Seer/Illusionist is the most powerful in game... Is a Red Mage + Summoner + Turbo MP and Double Cast combined + any form of MP restore without using items like Choco Recharge, Ether Cannon, Mantra Magik a good thing or very risky?
Illusionist/Seer is great for being able to double attack/cast a spell on the entire enemy team (plus it looks awesome). For Red Mage/Summoner, Blood Price is probably more broken since you don't have to wait to use any spells.

What does Mantra Magic do? I don't think I've heard of it.