1. Contemporary sources say Luleå was closed for 18-20 weeks a year, not 12.
2. There were other ports, too, mainly Oxelösund. However, the British Admiralty assessed in 1939 that this latter port could handle no more than 20% of the iron ore Germany imported from Sweden. It was mainly used for the shipments from other iron-producing regions.
3. From 1939 to 1944, at a time when the British where sending submarines and Coastal Command bombers along the Norwegian coast, both to directly attack freighters and to leave mines behind, the Germans kept using Narvik. On average, 20.1% of the Swedish iron ore going to Germany in those years went through Narvik. This implied, BTW, repairing those damages you quoted.
There's a technical solution to those problems available at fairly small cost - that is running icebreaker service convoys. Ships will need to be somewhat higher technical standard, but the problems won't be insurmountable at all.
That would, however, require some crash industrial effort and long term planning and some time, perhaps a year, all these weren't easy to do in Third Reich.