Hey there Vraudulus ^^
Let’s get to your questions point by point:
So for this first question we have two things to consider:
- The resolution set in the “Window Size” drop down menu
- The resolution scale set
The first option determines the effective size of the rendering window, and can’t go higher than what your monitor or tv supports.
The second option determines instead the resolution used to render the game within the above mentioned rendering window.
For the sake of making a simple and generic example, let’s say that you have the Window Size set at 1920 x 1080, with a resolution scale of 100%.
In such a case the game window will have a size of 1920 x 1080, and the game itself will render at the same resolution. Raising the Render scale to let’s say 200% (3840 x 2160) would mean that the game window will still remain the same - so effectively 1080p - however the game itself will render frames at a higher resolution, which will be then resized to match the effective Window Size set for the game.
So if the Window Size in your case is set with a 2k resolution, you’ll still be effectively playing in 2k, albeit the frames will be rendered @ 5120 x 2880.
Neither and both.
Frames will be rendered with a higher resolution (Oversampling) so effectively with more pixels, and then resized to match the Window Size.
This offers usually a relatively small boost to visual quality, noticeable mostly on edges and textures, at the cost of some extra processing power.
This can not however increase the amount of pixels your monitor/tv can show based on the related native resolution.
That’s unfortunately not something we can answer 
As Jingoraitin mentioned that looks to be due to some sort of pre-set option in your TV settings. We have no way to know which option or setting you’ll need to change or tweak though, so I am afraid for this specific point we don’t really have any suggestion, sorry T_T