Your perception of Finland is stuck in the 20th century. Today's Finland is roughly 10% immigrants. If the current trend continues, the fraction should increase to ~15% by 2030. That would be comparable to the US.

As for the Swedish-speaking minority, it's mostly a result of colonization in the middle ages. Swedish became the dominant language in some coastal areas, while the rest of present-day Finland spoke a variety of Finnic languages. During both Swedish and Russian rule, Swedish was used as the administrative language, and the elites used it among themselves. But even among the elites, Swedish was often not their native language.

> Finland is also just about the most ethnically, religiously, demographically and linguistically homogenous nation you could pick from.

Considering it has pretty much had effectively two primary languages for the past several hundreds years that seems like a stretch? Two of the most famous Finns of all time like Linus Torvalds or Mannerheim didn't even speak Finnish as their first language. Not exactly a sign of "linguistic homogeneity"..