It won't be completely stable, when used without a case. The geometry is such that on the side of the lens it's the lens that is touching the surface where you put it, not the bar-shaped bump. Meaning, it won't sit on the long edge of the bar.
Yes, see for example the hands-on video of The Verge.
You can also tell from the side profile images on the Apple website. Drawing a straight line from the lens to the bottom edge of the phone doesn't cross or touch the edge of the "plateau", by a good bit.
