I seems that your target was not parallel to the camera sensor and it was very close as well. At those distances even minor errors in focusing will show up, so don't worry about FF/BF for now, and DOF is so small that it will be really hard to judge anything.
To do a proper test I would suggest the following:
1) This is the major point: always use manual focus. The idea here is to test optical quality of the lens, not accuracy of the AF system. In addition, to avoid your errors in MF here is how to do it: (1) defocus completely; (2) focus manually; (3) take a picture; (4) defocus again; repeat the process at least three times (I do 5). Once done, choose the sharpest result. This way you will likely eliminate both AF and MF errors and actually test the lens.
2) To test corner sharpness take separate series following steps similar to those described in #1, but look at the corner target and focus there, NOT in the center! This is especially important at close distances. The reason is mainly field curvature and spherical aberration. For more details on lens aberrations see here:
Photographic optics
BTW: One of the key design differences between regular and macro lenses is that macro lenses are designed to provide so called "flat field" (translation: minimized field curvature and spherical aberration). So an average lens is even not expected to provide corner sharpness that matches center sharpness are close distances.
3) Finally, to compare color rendering set white balance to MANUAL (according to your shooting conditions).