What do you think is essential to make KC GL most effective in terms of feats and BFCs?

The number one most essential feat to give him is Trick Shot, because his biggest weakness is dealing with people in Stealth. The ability to shoot through other figures is just a bonus. Fortitude is nice, but not absolutely required (his best defense, of course, is his Defense). Force Field wouldn't be a bad choice in case of Poison figures, but they tend to pack Armor Piercing too, so it's not totally necessary. In Contact With Oracle is a great choice for everybody, but particularly so for Green Lantern since he has Willpower and can get the stat bonus 2 out of 3 turns... not to mention the ability to boost his defense even further is nice. Armor Piercing is not bad, since he has two targets.

Of the newer feats, Sidekick could be decent, and it looks like Protected would be a nice choice for him too.

BFC's- I'd recommend Bright Lights if for some reason you don't have Trick Shot. Darkness and Deep Shadows cuts his range, but his Running Shot can make up for that somewhat, and the benefit of always being in hindering (in Deep Shadows) can only help his high Defense. Earthquake and Atlantis Rising are of course always good choices for a flying character. And when all else fails, Ordinary Day works fine for him.