I'll let the poster you responded to answer the other portion of your comment.
But to answer your question:
The point of using a shots for/against (relative to teammates) measure instead of a goals for/against (relative to teammates) measure is to eliminate the smallish sample size over the season. Anaheim for example last year faced 3300 shots against and only 131 goals against. There's a much larger sample size with shots to extrapolate a consistent result from.
So from your example of the 30 sec shift - YES, that's a negative result. Key word you used was 'Clear the puck' <-- A big difference that can be tracked via advanced stats is how a D clears the puck. In the long-term, if every one of your shift is you stuck in your own end, getting the puck, clearing it and then changing... That's not a positive result. The D that get the puck out 'effectively' are the D that produce positive results. This can be done through carrying the puck out or clean breakout pass (short or long). Stralman IMO is the master of the short breakout pass. He'll strip the puck of someone and then make the perfect short pass that always goes to a F with plenty of open space to take it up.