Right, but you still need the red boxes and the general ASW level.
For example an IIb (def 1) attacks some loners (0 ASW) in the North Sea in WP1 (2 red boxes + 0ASW in WP1, [17.1]). Therefore the counterattack would be on the 1-2 line (2 minus 1) of the corresponding table.
No, because no sub that I'm aware of has a defense factor of zero. You subtract defense from ASW to find the strength of the counterattack. Less than zero means no attack.