I'm fairly certain you can. I know I've entered Epcot at 9 am through the International Gateway. They have CM's positioned to keep people flowing into Future World. I would imagine that if they're letting people in that way, they'd let them out too.
Yes, you can enter and exit at International Gateway from park opening until park closing. When you try to head back to the IG before World Showcase is officially open, just tell the CM that you are leaving the park.
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