Its possible (at least on paper) but will never happen.

To do this, they will basically be turning the switch as you know it into a glorified game cart. Its an easy thing to do though, just makes absolutely zero sense and would be a commercial failure.

Woops sorry, half composed sentence. Since edited to read:

"Nintendo are currently the only ones who can actually offer native 4k on their games with an affordable GPU"

Obviously not the chipset inside Switch but if they wanted to release a Switch pro, they wouldn't need to aim for 7/8Tflops in order to run BOTW in 4k, unlike with most Xbox One/PS4 games.

Nintendo isn't the only developer who can make game with an art style like BOTW... 

nah just a typo. when i hold shift sometimes the first tw letters in a word a capitals.

Its not about Nintendo as a developer or the artstyle they pick, its about the base specs of the machine they're developing for. Last Gen remasters like TLOU run at Native 4k on PS4 Pro because the core build and assets were built to make the most of a machine 1/16 the Pros GPU power, not 1/2 like ground up PS4 games. By the same logic games being built to run 720p/1080p on the Switch's 400-500Gflop chipset, will not need an expensive chip to make the jump to 4k. 

Of course developers could build their games around hitting native 4k on PS4 Pro but then they would be significantly under utilising the base PS4 model. 

No. The dock is just a housing attached to a charging/power cable.

It does zero processing outside the video upscale.

The hardware inside the tablet just performs better with higher voltage.

I say there is a 0 chance Nintendo would go down the road of multi GPU support on titles by offering a more capable dock. At that point they would just upgrade the Switch to the X2 chip.

Technically possible (just like it's technically possible to do this on a 1998 Thinkpad), however the performance gains would be so minimal due to the USB-C connection that it would never be worth it. You have a physical bottleneck that cannot be overcome by simply adding more power.

There can be but it is dependent on how Nintendo and Nvidia are doing it. 

For example, iphones have lightning adapters in order for them to charge but lightning adapters need to be certified by apple. If the lightning adapter is not certified by apple, the iphone will detect that and tell you that it is not certified by apple and I believe it will stop charging the phone in some cases.

So if the Switch requires a dock with a certain chip inside it that tells the Switch that it can now use all its additional power, then we can't have the docklike performance on any charger. However, if the Switch is designed to use the additional power as long as it is above a certain threshold, then we can use any adapter as long as it can push the amount of watts the Switch needs. And of course, since I am assuming you are talking about handheld mode with docklike performance, the Switch has to be able to detect that there is no HDMI out so it shouldn't turn off its screen.

So it is possible, just depends on how Nintendo and Nvidia implemented it.

I think it's possible, but super unlikely given how pricey the console already is..

No...they won't. The difference in hardware would eventually catch up to how the undocked switch plays games. It's not that simple.