Pure black n white with smooth curves

[Ofnuts]

Well, you have tried all my suggestions , so just trying to improve on method #2, starting with your result:

• Duplicate the layer and apply a double threshold to it, keeping only the grays (in my final test, the first threshold is lower than in the image below, around 70). Note that the thick black lines are converted to two thin parallel white lines, corresponding to the antia-aliasing pixels .
  

   

• Filters>Generic>Erode: this removes these lines and keeps the rest, you can use Filters>Generic>Dilate to get back to the threshold results, except where the lines were 1px wide, since they have now disappeared.
  
• Open the Channels list, right click any of the R,G or B channels and Channel to selection. You now have a selection on all the grays that aren't anti-aliasing pixels.
  
• Selection used for a Threshold: