Chain Mind controlling,
AA Mad hatter mind controls an opposing Doctor Destiny, and I take control of Doctor Destiny and use his mind control. He mind controls another character who is given an action and does damage to someone who is 100+ points.

Who takes the backfire damage, Doctor Destiny, or Mad Hatter?

I believe Doctor Destiny would take the damage. Mad Hatter would not take damage because his action was to mind control Doctor Destiny. Doctor Destiny's action is to mind control someone 100+ points so he would be the one to take the feedback damage.

The feedback is applied to the one doing the mind control against the specific target

So if you had your mind controller MC's the Fantastic Force's Professor Xavier (99 pts), for example, and you use Prof X's three arrows to the best extent, you can burn him for a lot, use the enemy's pieces against them and take 0 feedback!

MCing a MCer to MC others is awesome, especially if the MCer you are MCing is less than 100 points

Exactly what I was thinking, my friend runs Psychic teams a lot. I am going to get the Bruce Wayne LE this weekend, win or trade we'll see, and just have a 33pt F-U piece for fun.

this is exactly why i love MC. it is the best power in the game because it opens up every power in the game. i actually crippled a player in a fun game by using prof x with 3 /// and old school moon dragon and i kept MC'ing his pieces (2 of which were MC'ers themselves) and kept doing this turn after turn until he just gave up. fun times! :D

For extra fun use MC so your opponents take their own mystics damage. MC is an under used power in my opinion.

Of course, they have the option to cancel the TA.

This can work out well, too, as then you can attack the Mystic with no fear of reprisal.

MC a character, make them attack, reroll the attack with CSA, give them the token for CSA (because they are still friendly as their action is not yet resolved)

Power Ring finally has a use.

I've always wanted to fight against ff Prof X and 2 vet Karma's with an MC'er (I had a friend who played this team a lot, but I was inexperienced and didn't think of this scenario until my friend quit playing):

MC Prof X. Have him MC both Karmas and a Brick. Have the Brick attack someone on the team. Have 1 Karma MC Prof X and the same Brick. Have the Brick continue to decimate the team as you continue to mind control it and the other MC'ers. Loads of fun.

Actually in your example, the Prof is still friendly to your force as you have not yet resolved his MC action (Karmax2 is still under his control)

That's right, you can still do a good amount of damage in 1 action though.

The best MC trick I ever heard was during the days of soaring.

Prof X MCed KC Flash who was next to the AoE of the Aerial Baffler.
Move him to Soaring, move him into the AoE of the AB, gaining Earthbound. He falls and takes one damage. Move him out of the AoE, move him back to soaring, move him back into the AoE to fall again, and then keep repeating this.

I think you could "hit" him for 5 or 6 damage under the right circumstances.

I think Mind Control is the best solution to Norm's challenge nr 5, with the Runaways facing Superman Prime.
Nico uses Staff of One to get Mind Control, and uses it on Prime. Prime is then forced to cancel Invincible and move next to Molly, Chase and Gert&Old Lace. Molly clobbers Prime for 5 damage with the heavy object, putting him on click 6... which also has Invincible, which is still cancelled. Chase uses CCE to do another 4 points of damage, putting Prime on his last click. The attack activates Lend a Dino, letting Gert&OL finish off Prime. Down and out in three actions...