Hey! Nice for stopping by. Here are some clarifications regarding your questions, hope they will make some sense… 
First of all, it also matters if you are using scanline mode or not. If using scanlines and increased internal resolution, then you are good with Internal Resolution Y: 0.5...y-dowsample divider parameter set to the internal resolution multiplier. You can use X/Y downsampling for the pixel influence effect or increase Internal Resolution with the HD version.
You change the first parameter named Internal Resolution for the increased filter range. It’s WYSIWYG coded, so you don’t need to set it to 4x if to blurry, maybe 3.5 will do better for you etc. For scanlines it’s already explained a bit above.
This is a parameter section description, has no operational value, you must alter the parameter below.
High resolution scanlines enabled make sure that the vertical scaling component is also enabled if using a pre-scaler or increased internal resolution. Only valid when using scanlines and compatible with the Internal Resolution parameter with the HD version.
For the authentic buffer shrink downsampling it’s best to pre-pend a DREZ preset, there are many and serve the exactly this purpose. Normally shader parameters can’t control buffer sizes, so guest-advanced shaders do this visually while maintaining original output buffer sizes, but this can be slower.
With the ntsc preset you can lower the "NTSC Resolution Scaling" parameter to a reciprocal of the internal resolution (i.e. 0.5 for 2x, 0.33 for 3x, 0.25 for 4x…) and use the speedup parameter. Later on you set Downsampling-X and -Y paramters to 4x and add some neighbour pixel influence with Downsampling parameters.
Setting the Internal Resolution Y: 0.5...y-dowsample divider to 1.0 makes sure you are in the correct scanline mode regardless of the internal resolution or resolution changes which might trigger interlacing…
It’s a set-and-forget setting for users who want scanlines. It’s benefits are you can change internal resolutions and use it with different cores without changing the preset.
“INFO --> Internal Resolution Y: 1.0 - Smart Scanline Mode” is only an info line for the next parameter and how it’s working, it’s parameter values are here because even an info line must be a parameter and must have a value…
Many settings in the guest-advanced shaders have, as i mentioned, immediate visual impact, so it’s ok to play a bit with them, see how it goes and keep the most appealing setting.