Yes, Double Helix would've been on the short list for external developers that a publisher could go to for a medium-production-value game in established IP. In 2010, their previous projects included work-for-hire on Front Mission, Silent Hill, Star Wars, and many more.
No sorry, but this is bullshit. Whilst it's true that Double Helix would have fit the requirements for such a suggestion at the time... they came to absolutely nobody's mind prior to the announcement. This whole MS vs Sony first party exclusive "where will the games come from? they shut down everybody" dance has been going on for a long time, including prior to the reveal or launch of the XB1. Not once did I ever see Double Helix's name uttered in such a context, because nobody had them on any sort of radar back then and even if their name came to mind, nobody would have believed they could have been given an important project and not fuck it up. Furthermore, if prior to Killer Instinct's reveal I had said to you "Double Helix could be working on something"... not only would you have probably mocked the suggestion outright, but then you probably would also have pointed to the recently announced Strider as being what they were preoccupied with.
You can easily verify this by searching these forums. Prior to Killer Instinct's announcement, Double Helix's name wasn't uttered once (and neither was Armature's) in a Microsoft related thread. The closest any sort of prediction was that one thread where someone asked Not to be a jerk, but how do some of these devs keep getting work?
But sure... tell yourself you'd have called Double Helix developing one MS' more important launch titles back then.
By the way...
Creative Assembly - With Ensemble shut down, they're one of very few options that could've been hired to make Halo Wars 2.
Is actually a good argument for how unpredictable such a partnership can be. Halo Wars is the first project Creative Assembly has done for a publisher other than Sega since they were acquired back in 2005. The only reason why Creative Assembly would ever come to mind as a potential partner is because a Halo Wars game already existed, and you'd possibly try to imagine who could make a sequel. If Ensemble hadn't already made a Halo Wars game, no way in hell would you be thinking "MS could be working with Creative Assembly". By that logic, you'd effectively be able to call all of E3's announcements right now, because you'd know not only what the mercenary teams are working on (and how many of them are dedicated to each project), but also what any studio under a larger publisher is also working on.
Valve aren't a console platform holder.
They are a platform holder though, the lack of a device doesn't mean a whole lot. The first two PlayStations would have still killed everything else, just off third-party support.
If you want a closer comparison... then Apple's been doing pretty well too.