I give up, there is no way to end this. Did what Peni sugested, but I arrived at April 7 at the entrance to Narvik channel. Camped the place until April 20 and the only task force I saw was a.. huge convoy guarded by 6 destroyers heading to Narvik
They will always arrive there on April 12. I also saw a lot of brit merchant ships heading to Narvik even at April 20, this is just ridiculous.
I don't have any saves before April 5, so I can't restart the mission. And restarting the campaign is not an option. So I'm thinking to cheat and modify the save files so I can get the mission complete.. I really can't see any other viable option. This is a really bad mission design, not to mention that I don't get any help from Bdu or the whole damn german navy with hints about a huge battleship sailing in a hot area when germans are about to take Norway.