a) No, more active role, here in the structure in Norway:
https://de.wikipedia.org/wiki/Komman...eeverteidigung
https://de.wikipedia.org/wiki/Komman...gung_Stavanger
AvE put a list of vessels of the Flottille here:
https://www.forum-marinearchiv.de/sm...?topic=33885.0
(NS 23 is there listed , too --> see attachements)
Hafenschutzflottillen had a abbrivation for the area, they were used:
https://www.wlb-stuttgart.de/seekrieg/km/hsfl.htm
Their roles were harbour defence duties, picket service, escort of coastal convoys etc.
b) The netleyers you mean were named
NT (
https://www.wlb-stuttgart.de/seekrieg/km/netz.htm).
c) There were few mine destruction vessels (Sperrbrecher) with NS as name, but they served further south in the Channel area (
https://www.wlb-stuttgart.de/seekrieg/km/spbr1-8.htm)
Regards
Darius