Well..if we're talking about after NX, it's still possible for Nintendo to continue making consoles. Remember, it's been reported that they can survived over $250 million losses per year and it would be until we're seniors or something before we could event fathom seeng them bankrupt. As I said in my topic discussion on Nintendo and going third party, Sega was bleeding money before they dropped out with the Dreamcast. Nintendo appears to be in no position to do that since they still have billions to spare and IPs that are everything to them. If they go third party, though, I don't see them doing much else outside of Mario, Smash, Zelda, DK, and Pokemon since they're they only few franchises that can sell. You would probably get one FE or Metroid every seven or so years because they historically don't sell as well compared to the franchises I've mentioned.

In the end, they have to profit out of their consoles and software. Since they've been profiting with both Wii U and 3DS these part couple of years, what's not to say they can find a way to continue gaining profits? Especially since starting this spring, we'll see how their plan to expand their IPs will do for their future. With mobile, theme parks, expansion of merchandise, NX, possible TV shows/films, and other potential collaborations, Nintendo is trying to evolve into an entertainment business outside of their bread and butter, video games.

I would like to see Nintendo test the "waters" as a 3rd party and make a mainline new Mario game for the PS4, just to see what kinda support it would get from the sony fan base.

They go 3rd party and into the mobile market

From where?

They would quickly become the top third party studio and be extremely profitable.

I don't believe that the Nintendo needs to beat Sony or Microsoft with the NX, they just need to turn the ship around (which could be as simple as just selling 30 million units like the N64 days).  The new concept of the NX also doesn't have to something as crazy as the WiiMote.  Nintendo just needs to offer something that isn't being done by the other console makers and price the NX so that it can comfortably be a gamer's 2nd or 3rd console this gen.  I really think that, if executed properly, something as simple as the fusion concept could easily do this for Nintendo.

Personally, I always thought that a lot of Sony fans only liked the idea of having Nintendo games available on Sony platforms.  Kind of like how a lot of Nintendo fans only like the idea of having western 3rd party games available on Nintendo platforms.  Both groups in these examples want those games available on their platform of choice to help sales and for bragging rights but they're personally uninterested in buying those games.

Mobile will be the life blood for their console ventures if handhelds dry up. And really, that's all I care about. I just want to be able to buy next Nintendo consoles for years to come.

Well that's the trick isn't it? It's not so easy to come up with ideas like that. It's sort of like seeing a gorgeous girl across the room and saying "well I just need something great to say so I can walk over there". Nintendo didn't even invent the Wiimote, it fell into their lap because the inventor of the device was turned down by Sony and MS, but Nintendo was not his first (or even second) choice. 

Nintendo will be alright, even if the NX fails. Their most valuable assets are their IPs. Despite their hardware business performance, they'll be fine. They have lots of options such as going third party, partnering with other tech company like Apple or Google, or going mobile. However I don't believe they'll give up that easily, Nintendo would try again with a new hardware in the future.