Can you resize a window with only the arrow keys? For example hit r+up and raise the upper border upward, then s+left, and shrink the window by moving the right border leftward, or something like that?
Yes. After pressing Alt+space then R, the first pressing of an arrow key selects the border you want to move, let's say the left. You can then move that with the left/right arrows. If you press up or down in this state, it will, again, select the corresponding border to work on.