Preview line and half-page scrolling
I do not think line or half-page scrolling is possible. I'm not even sure that PDFs have, embedded, a concept of "line". I think that Preview tries to detect that for the purposes of selecting text (major improvements on this from 10.5 to 10.6).

You could set up a program (AppleScript, Automator, etc.), that simulates pressing the down arrow a certain number of times. Now, perhaps you click on the app in the Dock or run it via a keyboard shortcut, or even via a Voice Command, and it will scroll the amount you set it to scroll.