This happens due to the fact, that the width of the outlines is linked to the resolution of the image. So the relative size of them changes with the size of the viewport or with the resolution of your Renders / HighresScreenshots.
Unfortunately, this can't be changed. However, the best way to work with it is to have the viewport at the same resolution as the render output. To do that, first open a second viewport, then either use the console command r.ScreenPercentage or (in UE5) the Viewport Options to change the Screen Percentage. You should see the width of the lines changing. Also, always check the image in full screen before rendering. To give an example, if your screen has the resolution 2560 x 1440, and you are making renderings at 3840 x 2160px, set the screen percentage of the viewport to 150 to match with the render resolution. Check in full screen, and the look in the viewport should be the same as in the renders.
Yes. You have the choice to either turn the scene to grayscale, have it only white with the effects on top, or keep the original scene colors with the effects on top.
There is also an integrated toon-shader, with which you can give even a PBR scene a cartoony look. However, the effect works better if you've already textured your scene with flat colors instead of PBR.