Just wondering...

With regards to the multi-Mind Control strategy. How does the "no duplicate free action" rule comes into play with this strategy?

Mind control is the exception to that rule. Mind control is like the universal reset button. It's also why Mind Control can be used to activate 'once per game' or 'once per turn' effects multiple times within the specified unit of time.

Okay. If that is so, am I right to say that "Damage Depletion modifier" should not be an issue for this strategy?

This is an apples and oranges question. Mind Control lets you use repeat actions, modifiers are another issue issue altogether. I'm no expert, but I'm gonna say that the damage depletion modifier would still very much be an issue.

After an action resolves in which an attack was made, that character's damage is now at -1 until the end of the turn. Regardless of the fact that the attacks were made as free actions while under the effect of mind control. They are still actions in which attacks were made.

I hate damage depletion modifier. Useless and clunky mechanic.

Yeah...it is. Here's an easy way to tell if something falls under the Rule of 3.

Is it a modifier to a combat value?

If yes, then the Rule of 3 applies.

Mole Man+Dr Ock Arms!!!

He can mind control friendly characters, so one of your monsters can have an extra action as a free action. 2 attacks or a move and an attack in either order. Plus he gets a 12 attack for it and has Outwit.