ALL-OUT ASSAULT: Once per game, give Iron Man/War Machine a power action that places two action tokens on them. Iron Man/War Machine possess [1 target] for this action. They can make up to 4 close and/or ranged combat attacks as free actions. An opposing character may only be targeted once during this action. After resolving this action, Iron Man/War Machine are dealt 1 unavoidable damage.

1. If I Mind Control Iron Man/War Machine to use this, do I place the tokens or is this just a clumsy pre-emptive phrasing of double power action?
2. Can I Mind Control Iron Man/War Machine to use this if they have an action token on them?
3. The original owner can use this once per game, does the same apply per mind controller or is it once per team per game?

Can't it be both?

Yes, you place the tokens because the power tells you to do so. And, yes, this was a precursor to the double power action.

No. While the power action is now a free action, you are still told to place two tokens on the character. If you can't do that, they can't use this power.

Once per game per controller. This isn't really any different than using MC with other once-per-X powers (like, say, Outwit).

Can the Mind Controller choose not to use Indomitable to give pushing damage, as well?

I had thought this was ruled you could MC IM/WM each turn and use their AOA every time you MC them.

I was going to ask this question until I saw you already did. I think yes.

A couple points:

#1 - You can't choose not to use Indomitable.

Indomitable
This character can use Willpower.

There is no "may" there. Indomitable must be used.

However, looking at Willpower shows:

Willpower
This character may ignore pushing damage.

Thre is a "may" there. So Indomitabl (and by extension Willpower) must be used, but when Willpower is used you don't have to choose to ignore pushing damage. But Indomitable is always used.

#2 - Pushing damage always, always, always occurs as the current action(s) resolve.

In this case, that action would be the MC power action, not the free action given to IM/WM. And by the time MC resolves, IM/WM is no longer yours to control and has instead reverted back to its original controller's team. So you can't decide not to ignore the pushing damage; that would be the call of the original controller.

In regular English, 'can use' doesn't equate to 'must use'. 'Can use' is a very key dictionary example of an option, really.

If Indominatible read along the lines of 'Character has Willpower. This Willpower may not be turned off", then that would be rather clear cut for what you want.

Of course, Heroclix doesn't always use 'common English' so there's that.

It is clear cut.

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In Heroclix, "can use" does not include any kind of optionality.