When trying to train a player's low post, and only his low post, is it helpful to keep any minutes on footwork or will it improve more with just low post training? At this point the player in question has 30 minutes of low post, 20 for footwork.
Is it better to go way over the 20 minute max in one category or try to go to the max in multiple categories that help train one ability?

Couple of things.  25 is the max minutes in one category before diminishing minutes come into play.  So 30 is better than 25, but it isn't a full 5 minutes better.  Also, low post is just low post.  Footwork minutes work on defense and blocking.