While WAR is more fun to play and can gain more enmity than a PLD, you approached the later in a bad way.
Flash is your main source of AoE enmity, and it generates enough for you to keep aggro on all mobs if you use it efficiently. WHM or BLMs shouldnt need to sleep the mobs for you because you cant get the enmity. In fact Flash is good to gain enmity on CCd mobs by sleep because you dont even wake em up.
Also, a PLD at all lvls has enough MP to Flash at least 7 to 8 times in a row before running out of MP. Then you have your Riot Blade combo to gain more than half of MP that you need to use another Flash.
While very late, Circle of Scorn only boosts your aoe enmity when you reach lvl 50. As said, a PLD shouldnt have trouble with AoE enmity unless chainpulling and having wasted half of MP when getting a new batch without regenerating MP, but warriors can face the same issue, and even slightly worse because they depend solely on TP and have no skill to gain some back (they can still x-class Flash, but you only get 3 uses with full MP).
As for the OP regarding MT/OT question: the main tank is usually the tank that keeps the Boss while the off tank takes the adds that might spawn during the boss fight. An example would be the current endgame boss, in where the MT keeps the boss for 99,9% of the fight while the OT deals damage and takes the lesser adds. That fight has an add phase only were you end up dealing with certain adds and 1 in particular is assigned to the MT too for certain reasons.
There are still certain fights were tank rotation is a must, so Mt/OT in those fights is sort of irrelevant. That is also a reason that if you go WAR, you need to get the Provoke skill from Gladiator to perform the tank swaps.
