This is somewhat similar to the Eastern Gateway Project, phase 1 is currently happening, and that is to place the new infrastructure in place. The rerouting of the River, Train Tracks, etc along with sidewalk/pathway adjustments. This now allows the next phase to begin, which is the actual main work in 2017.
So is a 2018 opening possible, yes. But does it benefit Disney to open the new area during the current popular Halloween and Christmas periods, where the marketing money is focused?
More than likely, the answer is no, and why spring of 2019 is the most likely date, though that is truly a date that can be pushed back, as we have seen in the past. But look at Uninversal Studios Hollywood and what they did with the recent Wizardly World of Harry Potter. They actually got the project finished earlier than scheduled, and decided to have some special events including AP previews, and then soft openings to test the traffic flows and using real guests to get a better gauge on capacity per hour numbers. (The first testers have to be by California law, employees of the park and its contractors until the state officially approves the attraction for public use). Since employees now how to use the safety devices, they load more quickly on average than a regular group of guests. And then held the "official" opening events, including the major marketing in early April, matching up with the Spring date for Marketing.