I was wondering if you could use Mental Shields on M&M Jean Grey to avoid feedback damage.

She can be assigned the feat because she has Willpower later, and the card states "when the character uses Mind Control . . ."

I'm actually only thinking this would work if her SP and Willpower were on the same click, but I want to be sure.

She has to meet the prerequisites anytime you want to use the feat. If she's on a click with her Special Power (which does not meet the prerequisites) and she does not have Willpower either (I don't know her dial) then you cannot use Mental Shields at that time. You would need her Special Power and Willpower on the same click in order to use Mental Shields to avoid feedback damage from using Mind Control.

As long as Willpower is showing, you can use Mental Shields.

That's what I though, thank you!

Same deal with BiBtB Mandarin, he has Willpower or Mind Control for all but 2 clix. Very useful.