I agree, that is quite an impressive board for self etch. As you said ... lots of time and effort.
At this time my learning curve with Inkscape has allowed me to understand stroke and fills ... and use the x,y coordinates to verify positions of objects.
However the x,y coordinates seem to relate to the "edge" of an object, rather than the "center" of an object. Is there a setting that I have overlooked, or how can I change the size of the pads for the power connector from 3.5 mm diameter and 0.5 stroke to the desired size of 1.75 mm diameter and 0.35 stroke ... while keeping the center location UNCHANGED?
The stereo jack edit was easy ... ungroup and delete the two unwanted circles. Then save as a new file name and select from mine parts.