On the timing of announcements... a friend of mine just pointed out that Marvel Studios hasn't booked the main hall at Comic-Con this year. Normally they do a big presentation, and with the new Spider-Man you would have thought the coming together of the Civil War cast would have been the big deal.
So if it's not happening at Comic-Con, that almost certainly means D23 is where it's at. So it seems Disney is looking to push their convention as being much more than just a Disneyana-type fan event, but a rival, competing event on Comic-Con's turf and scale. If that's the case, it makes sense that they would want to save any decent park announcements for then too.