No one as of yet. The NX dev kits are a unconfirmed rumor. If they were giving out devs, it would be to anyone who would take one.

Square makes a decent amount of 3DS games still and if NX is really a unified concept that many speculate than Square will likely give solid support to NX.

Probably:

Ubisoft
Capcom
Square Enix
Activision
Platinum Games
Sega
EA (yes, I know, but they're probably gonna do some ports for it anyway)
Warner Bros.

If the API is fairly standard and the system specs don't contain something that will force devs to radically change their code just to get it to run - then the adoption rate from publishers should be decent. In that case it would come down to the commericals - what sort of incentives will Nintendo offer to get major support on board? (marketing support, access to Nintendo IP such as putting Link as an exclusive game character, a timed break on publishing royalties etc.)

You said 2017,  presumably fall. Which would mean dev kits were publically being sent out 2 years in advance which is not really a thing as far as recent history tells us. "Alternatively" you could be meaning an earlier 2017 launch such as summer, which would make handing out dev now quite reasonable (1.5 years) but a summer or Spring launch for a home console is not a common thing. So either way dev kits being out now points to a 2016 launch more then a 2017.

Well Square have 2 console defining games on the horizon (FFXV and DQ11) that Nintendo naturally want on their platform. Secondly the main reason why Nintendo have been ignored by Square since the 7th gen is due to specs.

Probably true. I'm just surprised they'd be all in when they've effectively done squat for any Nintendo home console since the SNES, and while their DS support was good their 3DS stuff would have qualified as tepid in prior generations. I guess it's not really a question you can answer, but I'm baffled as to why they seem more sanguine about the NX than anyone else, at least going by public comments so far.

They've largely ignored Nintendo since the 4th generatio on home consoles though, although admittedly they showed up to a respectable degree on the GBA and DS, and more than most on the 3DS.

I'll admit I was thinking of the NX more in terms of a home console than a handheld one.

Gotcha, but if NX does turn out to be both a handheld and a console that share a library than we need to consider the type of support Nintendo gets overall.

indies
lol

Sony for sure.