Question

Hello and blessings, Rabbi Michi. I have several questions on the subject of kilayim.
A. Is the required separation distance in kilayim (mixed seeds, mixed trees) like the case of meat and milk, where the prohibition itself is only when they are actually together, and the separation is just to prevent reaching that point—and therefore, just as if one still tastes meat after several hours one does not need to start counting again, so too here? Or is the prohibition itself defined by the required separation distance?
B. A practical difference stemming from the previous question: does one need to keep the required distance from the place of planting (assuming that even if the plant spreads a bit and encroaches on part of the minimum required distance there is no problem, since that is known in advance and included in the required measure), or must one always maintain the required distance from the outer edge to which the vegetable spreads? 
C. Would there be a difference regarding the previous question between the spread of a regular vegetable that spreads a bit (root vegetables) and a vegetable that naturally spreads significantly (pumpkin, cucumber)?
C. In our home garden we want to replace mint that is planted directly in the ground with another herb or with vegetables. The problem is that the mint has been there for many years, and mint is a spreading plant that is very hard to remove completely from the soil. Is it forbidden to plant something else there? How can one plant a vegetable there in a way that will not create kilayim? Does one need to replace or turn over the soil to the depth relevant for maintaining the separation required in kilayim? Thank you.