Emma can leave SB. She did tonight with August.
Maybe Mr. Gold (or someone else) wants her to stay. Whomever it is, prevented her from leaving the first time by having her swerve to not hit the wolf. They allowed her to leave with August but are preventing her from leaving with Henry.
Oh, and along the lines of leaving/not leaving, I took it that Henry grabbed the wheel & wrecked the car to stop her from leaving.