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Not obvious because some step drives list the microstep ratio:
2,4,5,8,16,32 etc but some list steps per revolution, 400,800,1000,1600 etc

OK, i have to ask, why FreeCAD after Inkscape?
If you are trying to do a depth map, none of those are good choices, try dmap2gcode from ScorchWorks website, it is very easy to use and works perfectly.
That is derived from the already included image2gcode that comes with LinuxCNC, so did you ever try to open a image file with LinuxCNC ? It works fine, but does require a bit more testing to get good results.

Have you checked the DIR output pins with a voltmeter when jogged in either direction?

A suggested step/dir daughter cards are: 

7I78 (4 differential step/dir outputs 1 encoder, analog spindle interface),
7I85S (4 differential step/dir outputs, 4 encoders, 1 serial port)
7I88 (8 differential step/dir outputs)

common parallel port breakouts:

Sainsmart 5 Axis CNC BOB
CNC4PC C11G
CNC4PC C10

 

1 piece props will require supports and that will require a lot of sanding and cleaning of all surfaces.
Multi piece ones are much easier and less cleaning and end up much more balanced.
A few nights back i had a light bulb moment, to print 1 piece props with no supports and at very low layer height so the next layer still has where to land, but i have yet to test how that ends up. I would start at 0.1mm for first test.