Hah, you crazy germans. That sounds like a fucking proper adventure, makes me want to do something similar.
Jämtland is a very popular area of Sweden for hiking, but bydalen is a bit hidden away in contrast to a lot of the popular trails and valleys, which I think is very nice. My family built a cabin along the river that leads to Dammån in the 60's, so the area has a special place in my heart.
Drommen and Höglekardalen are both very beautiful, and like you said, you cannot get clearer water than from the mountain streams.
I'm not too familiar with the area in Norway you are refering to, I've only been to Trondheim once. Perhaps some norwegian anons can help you out. Though the latitude is good for norrsken, we had a crazy show last year when we hiked in bydalsfjällen with very dramatic northern lights with clear green and pink. I only have shitty photos though,,
I did my first solo hike this march just upstream from Dammån. Pic related.