So I highlighted all the nice points you listed that further show that PLD is only a support tank and not a main tank.
You can pretty much forget any issues with MP on PLD. Playing as a support Tank, they most likely wouldn't run into MP issues as they are not constantly trying to hold hate and heal themselves.
SE - We will create a job system to give players a definate role during party battles
Players - Oh great
PLD ='s Support Tank
WAR ='s Main Tank
BRD ='s Party Support and buffs (DD)
ETC.