That's a TOTALLY different mythos, as I suspect you know. There's a lot of challenges with that script idea, from dropping an SFX bomb (city destruction) on top of an already SFX heavy film, two apparently unrelated villains, plenty of exposition, establishing a main character, then team dynamic (then questioning it, then correcting it) followed by implying the main character will become the villain in the next one? I don't think your core story is any more filmable than Johns'. It could be done well, but you'd have to get some pretty great writers. Hamm couldn't make that work on his own anymore than he could step in for Ryan Reynolds and make that work. I like the In Media Res thing though, though I do wonder about doing that with a film set on Earth, where there is a disconnect between the way things are after the Media Res, and how they got that way.
I would have done it a bit simpler:
Hal is a guy with immense potential who has squandered his life, can't get it right. Living off family, drinking too much, can't keep his relationship with his boss/girlfriend, but he's good at being a pilot, though he's a bit lit when he does. Gets this space cop ring and speech, warned a trainer will be coming, abuses his power and gets pwned by Hector Hammond/Legion/Whatever Harbinger of Parallax You Want. Meanwhile, Parallax builds in the background on a path to Oa which happens to go through Earth. Sinestro appears to train Hal, recover information, lets him know that Parallax is coming, a being that is raw fear incarnate. Hal goes to the Guardians to seek help, we get glimpses of how busy and cool the Green Lanterns are, the Guardians refuse to see him, he barges in, they won't help, he goes back to Earth and has to get his stuff together, from the girlfriend, to the piloting/his job, to his family, and he masters willpower. With his closest confidant (probably Carol) we get to the bottom of his fear, and why he fears being successful. He doesn't want to overrun and tarnish the memory of his old man. Of course, the Parallax arrives, wipes out some people in epic disaster movie fashion, Hal powers up, perhaps some help from Carol in a jet, or Tom with the drones or something, and basically overcomes fear completely, making himself invincible to Parallax, making the people who see the event less afraid, throwing the monster off balance, at which point Hal, in a airplane construct totally outflies, out guns and outshines Parallax, leading him away from his target until he uses some Chekov's Gun to finish it off.
Basically, like the film, but with character interaction instead of heady Corps exposition and focus on Oa, but more focus on making him a superhero, and the relationships with his friends and family, and his development.
