2.) Submissive Affliction. Is the controlled enemies' attack on it's ally being modified by their attack modifier deck?
player's monster's attack modifier deck.
4.) Feedback Loop. The shield effect gets activated after the MT attacks in a given round, right? Not everytime a enemy hits?
Yes, see official FAQ on Mindthief.
5.) Mind's Weakness. Does the attack that is activated with this card immediately benefit from the buff that Mind's Weakness bestows? So is it a Attack 3 immediately?
If it's considered a persistent bonus, then I would say yes. See official FAQ on Mindthief and pg. 25.
3.) Fearsome Blade. Does the card net the Mindthief it's XP bonus even if the target is not hit / can't be hit / wasn't damaged?
Are you not within range (melee/adjacent) to hit it? Based on pg. 27, you can only get experience if you perform one or more of the listed abilities on the card. For this card, the attack and push are both melee range, so even if you don't do damage, you still apply the push if you're within range (pg. 22).
I recommend reading the rules again .