If they can make a highly successful, well done origin movie for Iron Man why can't they do the same for Flash/GL?
I'm not saying it can't be done. But they can't hold anything back. They can't be thinking "Well, if we cover Hal's origin in part 1, we can slow-boil up to the real action in parts 2 and 3, then the REAL epic stuff can happen in the second trilogy!" I think with a Green Lantern movie - and with a Flash movie - they'd need to go all-out balls-to-the-wall with part 1, THEN think about how they can top that with a sequel if/when it's greenlit.
That said, I wouldn't mind if they did some continuity-compressing for a "Green Lantern" movie. Namely, start with Hal Jordan's origin, an opening scene detailing the plane crash that killed his father maybe, segueing directly into Hal as a pilot. Quick intro to the supporting cast, then launch into Hal being taken to Abin Sur's ship. Have him become a Green Lantern in the early going of the film.
Then, you can devote the rest of the first act to Jordan's GL training in Oa. For the sake of plot efficiency, I'd probably merge the happenings of Emerald Dawn and Emerald Dawn 2, and just have it be Sinestro that trains Hal rather than Kilowog. Then you can directly go into the revelation of Sinestro's activities in Korugar, and Hal's "betrayal" of his mentor by reporting back to the Guardians, leading to Sinestro's banishment.
With the film's second act, you have Hal Jordan taking up his duties as a fully-functioning Green Lantern, perhaps keeping him occupied with a minor, Earth-based villain. Meanwhile, you have Sinestro discovering the Yellow Power Battery on Qward, and gathering together his Sinestro Corps.
Then the big finale that dominates the third act is a big-ass space war between the Green Lantern Corps and the Sinestro Corps. It's a lot to put in 1 movie, and it'd probably run north of 2 hours, but it would pack the film with enough epic action and drama to all but ensure a sequel.