Turn out the new method to generate border proven to be much much simpler and you can use full collision shape for the tile. Thanks to "just enough" collision shape of the border sprite, previous censor isn't required but rather using the border itself as a detector. Now it's variable free: dropbox.com/s/0c08ihjdzy3an3q/efficient_pad%20border_area.capx
If only I discovered it earlier. Nonetheless it's a journey for better cleaner blocks of code.