I think that:
since for a "normally" working user (i.e. one that types in his/her answer without any major no-typing-breaks) there's no need to display anything (like you said, in this case, showing a non-counting counter would be just confusing) then, don't display anything
only show the counter when a no-typing break duration has been reached. This should be fairly simple, maybe a circle that diminishes (i.e. a "pie-chart" that loses slices, etc), eventually with a working senconds counter underneath that counts down. There's the "Attractive jQuery Circular Countdown Timer Plugin - TimeCircles" that does something like this:
Maybe to the side a message saying that "you should type in your answer in a timely fashion, or cancel it so that others can use the slot".
- I think the most important rule to convey here is the one where the timer is reset if the user re-starts typing. The user, once shown a down-counting timer, should understand that once he's restarted typing, the counter is re-winded, he's not put in a race against time simply because of one typing break.
I think that once you get to this state (the counter was shown, hasn't counted down to 0 yet and user has restarted typing), you should do a simple, quick animation, showing that the counter is "re-winded" : if you display the seconds counting down, make them count back up to the initial value real quick, or if you use some visual aid to show the time trickling down, "revert" the animation to its initial state quickly, etc. Then leave it there for another second or two and then make it go away. I believe this give the right idea of "now that you've restarted typing, the counter has been reset and it won't be an issue unless you make another similar break, at which point it will start counting from the initial value, not continue from where it left off on your previous break"
- Finally, I think that the only instance where the counter, once shown, should remain visible forever (i.e. until user manually dismisses it) it's when it has actually gotten down to 0. In this case the user may have actually been AFK, and he should see what has happen in the mean time when he returns