Hi Tim,
I've canoed in Dalsland a few times, in early May and late September so at either end of the boating season, it was very quiet at both those times but I believe it gets pretty busy for the two month summer holidays when Sweden closes down and everyone goes canoe tripping (so I'm told!)
weather was generally settled but we had some cold and heavy rain as well as moderately windy weather. I think it's pretty warm in mid summer.
I think it'd be a great place for a swallowboat, the distances coverable on inland waterways are mind boggling coming from Uk.
Mark