Just wanted to make sure this works before I try it and get overruled...

Can [Crisis] Green Lantern effectively use the Shellhead feat?

I assume that he can, based on his [SP] which grants him use of Toughness,
Willpower and ES/D.

Can I get a confirmation on this please...

Definitely. Far better on the BN Hal, though. Shellhead and pushing to the 2nd click puts him at 20DV. When you push and heal off that 2nd click to get to the 1st click, he's sitting on a 19DV.

SWEET!

Yes he can! Remember the Rule of 3 though, the most he can get is +3DV from ranged attacks!

Just remember two things:

1) Because of the Rule of 3, Hal's final modified DV will still only be +3 of whatever it normally is; the +2 from Shellhead and the +2 from ES/D will max out.

2) He'll be very, VERY vulnerable to Outwit. It would be a good idea to Outwit his special power normally (to get rid of the ES/D, Willpower, and Toughness)... now, there is even more incentive to counter that special power and remove the feat's prereq, too.