I can't think of where this would be...If player 1 Mind controls player 2's Mystic, then player 1 turns off Mystic TA, does the Mystic TA return when control returns to player 2?

no the power is off until the end of the turn.
This is also useful against Power C and Q TA's cancel their TA's then outwit that def to deal damage, but the TA will return and all powers will return when the TA returns.

Also note that canceling the TA is not an action, so you can cancel it and then still do something with that Mystic.

The right sequence of action :
1 take control of the two mystics
2 give a free action to the first one
3 before resolving this action, change the TA of all your wildcards into mystics (now that you got one friendly to your force); CANCEL IT
4 attack a non-controlled character. If possible, another mystics ! Your opponent will then have to choose between cancelling this new target's TA or watch his own character take damage because of it !
5 give an action to the second one ; cancel his TA
6 re-do step 4
7 blow away the second now not-so-mystics character with your heaviest hitters, but make sure the first one (whose TA's you are using) stays alive.
8 laugh madly at your opponent

EDIT : correcting the post based on Harpua post

There are a couple of mistakes here.

1) The first guy can't attack the second guy. At that point, both are friendly to you and each other.

2) There's no reason that you cannot copy a canceled TA.

Corrected.
Thanks Harpua (beginner mistakes, I'm ashamed...)

Dang, never would have thought of this, thanks guys. Time to plan out my next Scientist team with Doc Destiny...

Ya Mind Control kicks ace!!!

You leave the Bat-Hound alone, you animal abuser!

One question I've had for a while:

If you give a free action to a character you've MC'd, like perplex or outwit...does that count as your action for them or can you still give them a non-free action?

Yes, a free action counts as an action. MC allows you to give an action.

Cool i now love mind control even more