I agree with Mackguyver. In order to comment on corner softness or misalingment of a lens element you should be 100% sure about the following:
i) the subject should be flat (in your case the leaf does not seem to be flat)
ii) the sensor should be perfectly parallel to the flat subject
The other test pictures shows no significant variation among the corners; so if I were you I would not worry about the lens!
For the best results i think you should also consider diffraction limits (before going to a setting like f20):http://www.luminous-landscape.com/tutorials/understanding-series/u-diffraction.shtml
If you go beyond f11 the image loose sharpness due to diffraction effects!
As a rule of thumb f8 (maybe f11) is the optimum setting you should use. If you use a smaller aperture you may increase the depth of field but the image sharpness will degrade.
In macro photography, if you want to get your subject completely in focus you should try focus stacking:http://en.wikipedia.org/wiki/Focus_stacking
(here you can find some software solutions to stack the images)
You may need a macro-focus rail (and patience!) to get good results;
Good luck and happy shooting (you have a nice lens)