From my understanding of the schedule, a 'demo' opening between the underground stations is quite feasible on July 1, 2017 if they make good time, but a full system opening is much more difficult.
The schedule calls for the downtown tunnel to be more or less finished by summer 2017 with construction work winding down significantly, meaning the tunnel with all the track, signalling, electric, etc. could easily be service-ready by Canada Day 2017 if they make good time. But uOttawa-Hurdman & Lebreton-Tunneys will still be in construction, as those segments are starting later to minimize Transitway disruption. Hurdman-Blair will also be ready fairly early; in fact the plan calls for Hurdman LRT station to be completely finished by the summer of 2016--meaning we'll have a ghost station sitting there for 2 years.
While the tunnel could be easily ready for Canada Day 2017, the trains are a bigger if. The vehicles have to be assembled on the line due to their massive size; the parts are being manufactured in New York, and then they are going to be shipped to Ottawa and assembled at the maintenance yard, construction on which is only just starting now. Because they have to build the maintenance yard first before they can even start vehicle assembly, the vehicles aren't scheduled to be ready until winter 2017-18.
It would be interesting if they do open a demo service for Canada Day 2017. It would be the next best thing to an early opening. There's no short-turn points anywhere inside the tunnel, so trains would have to restrict themselves to a single track in the event of a demo opening. It could conceivably consist of a single train starting at Rideau Station's westbound platform, heading down the westbound track to Parliament and then Lyon, and then switching direction at Lyon and running reverse-direction along the westbound track back to Rideau.