Short answers, pulled from the story.
Speakers of the North Germanic branch became distinguishable from other Germanic language speakers around the year AD 200. This separation occurred during the Migration Period which lasted from AD 300 to 600.
North Germanic languages serve as national languages in Denmark, Finland, Iceland, Norway, and Sweden. Finnish remains the majority language within Finland despite being part of the unrelated Uralic family.
Approximately 20 million people speak a Scandinavian language as their native language across Nordic countries. The total speaker count reaches approximately 24.3 million including second-language learners mostly located in Finland.
Norwegian speakers demonstrate the best ability to understand other languages within the group according to various studies. Faroese speakers scored highest among all groups at 7.01 average despite being taught Danish in schools.
Sweden left the Kalmar Union in 1523 creating two separate Scandinavian units ruled from Copenhagen and Sweden. The Denmark-Norway union existed until 1814 when it was disestablished leading to different international contacts.