But it seems to me that the "c" in "defensive" produces a different sound than the "s" in "defensive", whereas the "c" in "defence" and the "s" in "defense" sound the same.

This does not make any sense. Both defense and defence are pronounced with the same voiceless alveolar fricative /s/. We can't really argue about the non-word defencive, but if it were a word, I can't see that it'd be pronounced any different than defensive. Just like the 'c' in civet.