1 - Thief could steal TP, HP, MP, aggro, etc. Doesn't have to be items. Thief and Ninja could be drastically different, stem from different archetypes/classes (I don't know much about weaponry, but wouldn't a Ninja be more apt to use like a pair of blunt objects like MNKs do rather than dagger like a THF would?) and do very different things.

2 - I think an evasion tank could work as long as you combine it with a parry tank. i.e. have the tank stance Parry EVERY attack and increase the % you parry to make the eHP PLD/WAR equivilent, then have all its "CD" abilities be evasion based. The main one being something like Kitru's Ninja had in a 3-5sec ability with a 30s CD that evades and counters (with extra enmity) every attack in that time frame. Any point that says "They can't do that because of Game_Mechanic_08" is invalid because they can break the rules of the game by changing it any time they want, and said that they would previously (i.e. changing enmity modifiers on tank classes to turn them to DPS). And don't hyperbole the crap out of that statement that I was using rhetorically, obviously they can't change certain things, but within reason they can do whatever they would like to do.

3. Corsair into Thief AND Muskateer.... eh? eh? EH!?