Their WiiU might be selling poorly, but on the other hand, their 3DS handheld is dominating hard and doing fantastically in hardware and software sales.
I wonder if Sony or Microsoft might not cut Nintendo a deal for putting Nintendo on their consoles; but I don't see why Nintendo would want to sell their IP at $10 or less a game on iOS and Android, when they are currently selling them for $40 a game, quite successfully, on the 3DS.
Yeah, yeah, there are billions of iOS and Android devices... but not all of them are smartphones, not all the smartphones are good, not all the smartphone users are gamers, not all the gamers are interested in Nintendo's flavor of games (though Nintendo could very easily adapt - gameplay-adapting to devices is Nintendo's specialty), and Nintendo would be fighting for store positions against Angry Birds and other such games, would give up 30% of the revenue, would lose the revenue cuts other developers currently pay Nintendo, would get pirated like you wouldn't believe, wouldn't control the hardware (a long-term important key to Nintendo's success up to now), would be developing games across a configuration of hardware instead of a consistent platform (something I think Nintendo would have a problem adapting to).
As you quoted yourself: "It’s not as simple as enabling Mario to move on a smartphone."
Nintendo dropped the ball majorly on the WiiU (as badly as they did with the GameCube). I think it'd be financially better for Nintendo to cut a deal with Sony for this console generation, while developing better plans for the long-term.
Microsoft wouldn't want to put the xbox at risk, but they do have their own lines of tablets and mobile phones. Unfortunately they don't really own any good game IP directly. They could work with Bungie and get a Halo game, or work with Lionhead, Twisted Pixel, or other studios they bought and update some of their older properties, but those are all long shots.
[Edit:] (Bah, completely skipped over the key qualifier, "studios they bought" - all the below was based on me missing those three words. )
While Nintendo is the king of IP, and Microsoft doesn't even come close, Microsoft does have some IP they could be doing a better job to leverage to their advantage, whether to push the XBox or to push WinPhones and tablets.
Microsoft doesn't have to work with Bungie - Microsoft retained the Halo IP after Bungie bought back their independence. Now Halo is developed by an internal Microsoft studio (343 Industries - created by Microsoft solely to manage Halo). That leaves Microsoft perfectly free to make Halo smartphone games to try to motivate people to migrate to WinPhone8.
Another good IP that Microsoft owns that would translate well would be Age of Empires (hey, I own a Microsoft-approved Age of Kings port for Nintendo's DS console. Pretty poor quality though).
Other than those two games, Microsoft owns a few game studios that have a little bit of IP that might be worth using to promote Win Phone 8: They own Rare, and thus, Banjo and Kazooie, Perfect Dark, and uh, Conker's Bad Fur Day (not sure how popular that one is, but I've at least heard of it before). Forza Motorsport I guess, and Viva Pinata (Viva Pinata might translate its characters well to small mobile games).
You mentioned Lionhead Studio (Fable, Black & White) and Twisted Pixel Games - both are wholly owned by Microsoft (1st party studios).
Microsoft might also own the IP to Lost Odyssey and Blue Dragon. They partially financially funded the startup of the development studio, then they (Microsoft and the studio) split up and Microsoft possibly retained the rights.