I've been lurking this thread all day and finally want to chime in,
I think the idea of Zelda and 3D Mario at launch is....insane. Twitter rumours be damned, it just isn't smart on Nintendo's part of overshadow 3rd party releases. It also allows them to continually polish one of them and give it a dedicated release window with the proper marketing.
It's not that they couldn't market both at once, but a big 3D Mario launch in summer or fall, with lots of website press, journalist hands-on, reviews, pre-orders - it all just helps the Switch's overall mindshare.
And with the amount of Zelda we've seen in the press (E3, VGAs, Jimmy Fallon) it makes sense as a launch title. As others have mentioned, it's also a key title for a more hardcore, early adopter audience where 3D Mario is better with kids, families, and Christmas.
With that said, I think it's more than a safe bet that Nintendo will have more than one 1st part launch title, and one of them most certainly will include local multiplayer to showcase the multi-joy-con co-op. I strongly predict that. And, in all honesty, I'm surprised Mario Kart is not the obvious choice for a launch title in that regard.
Mario Kart makes perfect sense. The MK team has had enough time to enhance MK8, MK is the biggest seller on WiiU and Wii, with lot's of mass appeal and brand recognition. It shows off the new joy-con multiplayer, and maybe they will sell a bundle with the car-adapter thing? Honestly, MK8, Zelda (and Splatoon? only because rumoured) are the perfect launch titles, and they leave some breathing room for third parties.