If the attacker wins an assault across a wide approach but chooses the assault to be narrow attack do the defenders still take wide approach damage?
Yes. Infantry in reserve (and not involved as defenders) take retreat losses according to the actual width of the attack/defense approach.
Infantry and cavalry in other approaches take retreat losses according to the width of the approach they occupy.
Retreating artillery anywhere, of course, are eliminated (and don't count toward satisfying retreat losses in the other positions of the locale).