So a couple questions for TheXRavenX, but he hasn't poked his head in for a week so if anyone knows the answers please chime in.
a) I was waiting for the Steam Machine ver, then I saw the Hivemind interface which looks really rad for someone that uses GOG, Battle.net and will have to use Origin for Battlefront, but I still kind of want the Steam Machine because of the value (I have an authentic copy of Windows sitting right here) and the second (for me) Steam controller. Will there be a way to dual-boot the Steam Machine and install the Hivemind interface on the Windows partition? That'd be super awesome. And...
b) Will the Steam-branded box be sold anywhere aside from Gamestop? I'd rather buy it pretty much anywhere but there, plus I have a lot of Amazon credit.
You can dual boot windows and steam os, there is an option to do that when installing steam os in the first place.
Hivemind is just a skin for kodi, formerly xbox media center. They are launching steam and origin and such from kodi. Personally, I prefer the other way around - I boot into steam big picture mode on my windows machine, then launch origin games from steam. And then, if I need to, I can launch kodi from steam to watch movies.
You can launch gog galaxy through steam the same way.
gmg, Amazon etc are just websites. I have a shortcut to firefox that opens into those websites as icons in steam. That means, in steam, I have a button that says "greenman gaming" that opens a browser to that site.
The benefit of running through stream big picture mode and launching everything through it is that the steam interface follows even to non steam apps. So if I launch, say, kodi, or and origin game, then pressing the guide button on my controller still brings up my steam overlay, which includes the friends list and voice chat, and the integrated web browser.
But to answer your question - yes you can do that. It's a pc, you can do anything. You could also, say, ditch steam and hive mind entirely and go with something like hyperspin:
https://m.youtube.com/watch?v=mSSfNwUieAc
Or use ice to integrate retro arch into steam. That's the beauty of an open garden - you can select your shell. In steam os, you can even do things like integrate with more exotic things like mythTV.