OK, so we know that a white power gives a figure the USE of a power/ability, but the fig doesn't actually have it. This would usually mean that a feat card couldn't be applied just based on the white power. (Hawkeye's smoke arrow/smoke bomb and flashbang, in example.)

What if a fig, on a different part of the dial, HAS a power that the white power normally lets them use? This is in reference to Mandarin, who has regular Mind Control on parts of his dial AND his white powers let him USE Mind Control.

The big question is: If I give Mandarin Mental Shields, would it apply while he's on his white?

In Mandarin's case it would on *most* of his clicks, the reason being that he also has Willpower on almost all of the clicks where one of his hands lets him use Mind Control.

Willpower allows him to use Mental Shields, and thanks to his special power, he can use all of it instead of just the immunity to Mind Control. However, his special power has nothing to do with his ability to use Mental Shields, that's ALL due to Willpower being on his dial.

No Willpower or Mind Control (the actual power) showing on the dial = No Mental Shields.

I hope that clears things up for you?