There is a very tiny plastic tab, on the front of the camera, right in front of the GPS cover, on the bottom right. You need to pry it slightly forward, then the whole top will come off.
We had already sort of inferred, that some the display area on the JDI display they used, was hidden/masked in order to give the same size screen and resolution as the classic Pebbles. But after seeing exactly how much of it you're losing - it makes me sort of wish for the whole display to have been used/exposed for PebbleOS 3.0 and SDK 3.x apps, and instead display "classic" apps in a partial screen with a border. That would have helped reduce the bezel complaints too. :-(
It's awesome that the buttons are on the mainboard now, and not on the ribbon cable anymore! They also look to be actual micro-switches now, instead of mylar dome switches. Both changes should eliminate inconsistency in button feel (between identical watches), and increase button life (buttons wearing out/sticking/getting mushy, seemed to be one of the more common RMA issues, as is common with mylar dome switches).
I find it very interesting that Pebble switched from an enclosed pancake style vibrating motor on the classic Pebble, to a more traditional open weight cylindrical motor. I wonder if this is to make vibration more tactile, and less audible?
Yes, it has a headset jack, you can use a mic
It does in fact, have a headphone jack that supports headsets / microphones.