1. Both/All UBR stocks have an integrated recever extension, so yes, it comes with the stock.
2.Any collapsible stock will, or can be made to, work on a 308AR.
3. "Any carbine-length receiver extension for an AR15" will thread into your lower receiver. However, at that very point, that you mount that part up, you need some shortened buffers. The Armalite AR-10 Carbine receiver extension is the best way to go, because it's longer than the standard AR15 carbine receiver extension, AND it used regular old AR15 Carbine H3 buffers.
4. Mil-spec. Hands down. No need to run the Commercial when you have the choice.
5. No, that's just the copy of the DPMS-based parts, and the short buffer.
I alluded to this before, in one of my earlier posts in this thread. Eventually, when the UBR2 hits the market, you KNOW you will change that backend out with the UBR2. Make it easy on yourself now. Standard AR15 carbine receiver extensions have an internal depth of 7". That forces you to use a short 308AR carbine buffer. The genuine Armalite AR-10 Carbine Receiver Extension has an internal depth of 7 5/8", which allows you to use the longer, Standard AR15 carbine bsuffer - in a 308 build, you use the H3-weighted heavy buffer. Use the Armalite EA1095 spring with this setup. You can purchase the necessary parts in one place, right online at DSG Arms.
Once your new UBR2 arrives, your spring and buffer will drop right in and function like they're supposed to - in the UBR2.