Yes, most iPhone SE models are unlocked.
Yes, the iPhone SE has HD Voice capability.
Whether or not the iPhone SE delivers full wideband audio during a phone call depends on a variety of factors. First off, the availability of HD Voice depends on what network technology is deployed by individual carriers. Even carriers that support HD Voice may have only rolled out support in particular markets.
Yes, the iPhone SE has the same 12-megapixel iSight camera as the iPhone 6s and 6s Plus. The iPhone 5s is only equipped with an 8MP iSight camera. The iPhone SE also includes most of Apple's latest camera technology, besides optical image stabilization. These features include 4k video, Focus Pixels, support for Live Photos and more.
Since Apple introduced the larger iPhone 6 and 6 Plus a few years ago there have been a number of 4-inch diehards who have refused to upgrade. It has been unclear whether Apple would completely abandoned the 4-inch size, but Monday's event made it clear the 4-inch form factor is here to stay, at least for now. The long overdue update to Apple's 4-inch iPhone form factor is finally here.