You know, I wouldn't be so sure on those either... Remember the game (probably) isn't even coming this year. Plus, even if they did announce it so far away, I wouldn't count on it being the final date.
They will announce a release window at E3 at the very least. Release windows are announced at times an entire year in advance, that could be anything from a season with a year to even just a year. So, unless FFXV isn't coming out until 2017, we will hear some release window from E3.
As for it being at the end of the demo... no... Like you said, they're not just gonna announce something that BIG in a random small time live stream, and they're not going to put it in the demo either because the purpose of announcing the release date is to tell the entire world. Of course, the internet would report on it, but then what's point of E3?
We won't get a release date from the demo, or at this ATR. It will be at E3, and it's likely just going to be something like "Q? 2016" or something like that, no specific date.