I'm working on a post-jam update where I switched back to using A/D for steering. The issue with using it at the time was there is no delay from when you are walking to when you are drilling. So if you're walking right(D) and you begin to drill you'll immediately begin to nose dive. Which I found myself having to correct my path faaaaaar too often than it was worth. As I said, I'm moving back to the A/D scheme in this post-jam update because I've re-worked how some thing operate.
Yes, the drill zone are basically a bunch of 16x16 tiles. The drill zone makes use of Quad Tree data structure so if it gets hit, then it'll break down into 4 smaller pieces (1 in each quadrant). Once they get down to a minimum size, I just delete them.