I think it's Gorilla Glass 5 on the front, 3 on the back, so not a shatterproof polymer. (Source: if I remember correctly that's what the HTC guy said on a video on ars technica).
The HTC US site's order page has the text "The
unlocked HTC 11 is compatible with AT&T and T‑Mobile Networks." on the pre-order page, above the order button. So hopefully that does mean fully compatible but yeah, if you have a specific requirement like T-Mobile VoLTE it's probably worth checking explicitly.
I worry about the ergonomics. 5.5" 16:9 is bigger than I'm comfortable with, and it looks like they've not helped by bringing the capacitative buttons in towards the middle, so they are both far out and very low down. Since I struggle to reach the back button on the 10 I think this is likely to be a deal-breaker for me.
As for the DAC in the dongle, I'll be surprised if it's not the same as the Bolt/Ultra dongle. Sadly all I can tell from their site is that it has the same price tag