The Tornedalen people are kinda their own group with their own language (Meänkieli). To an outsider it might sound like Finnish, but it's not much closer to Finnish than Yiddish is to German.

Meänkieli split from what became the Torne Valley dialects of Finnish in 1809, when an international border suddenly appeared in the middle of the region. To a Finnish speaker it sounds like a bit old-fashioned dialect, but it's mostly intelligible. It's definitely closer to standard Finnish than Karelian, which takes a lot of effort to understand.

Not so sure. As a native Finnish speaker, I'd say most of us would understand Meänkieli just fine, especially anyone raised on the coastal areas where colloquial Swedish loans are more common in general.