What if Nintendo intended it for release in mid-2016, but bumped it up to holiday 2015 to compensate for losing Star Fox from the holiday season?

As for why not specifically announce it earlier than this? Simple - Zelda is a franchise that gets a dedicated announcement (not a press release or a random trailer upload), so it would have to be in a Direct. But Nintendo haven't been ready to start back up with the directs, yet. If they'd known that they would need it for 2015 at the time, they would have revealed it at E3.

Or maybe not even then. No point in taking attention away from Triforce Heroes, so they can wait until November, and reveal it in a "And it will be available in XX days" kind of way. If it looks even remotely like the Zelda tech demo (and I could see them working to get it looking something like that, since TP is a more "realistic" style Zelda than Wind Waker or Skyward Sword), it won't need much extra push to sell well.

While a spinoff isn't entirely implausible, I can't see them doing it right now. There's no benefit to doing so. They were experimenting with art styles in the leadup to deciding on what Zelda U would look like, and that's where Wind Waker HD came from. I expect that Twilight Princess HD came about in exactly the same way, and that it's been in development since Wind Waker HD released, in parallel with Zelda U.

Heck, I bet there's a heap of references to Zelda U in Twilight Princess HD, in the same way as how Skyward Sword artwork was added into Ocarina of Time 3D.