F16 is way past the point where diffraction has kicked in.
I'm not saying it is or it isn't softer than you should expect and I understand why you want to increase DoF at cost when doing macro, but at this f-stop point you're not testing the lens, rather physics outside its control.
Question 2 is - how sure are you that the subject is perfectly parallel to the sensor plane? At the focal length you're using minute shifts away from a perfectly coplanar situation will appear as softness, and be noticed first at the edges.
AND judging by the fact you are soft up top and sharper as you go down the image I suspect this is a problem with the test shot provided.
The closer I look the more sure I am that you are not seeing softness as a result of the lens, but rather a subject tilted in relation to the focal plane.