Short answers, pulled from the story.
Approximately 20 million people in the Nordic countries speak a North Germanic language as their native language. Swedish is the most spoken of the group, with around 13,200,000 speakers including second-language speakers, mostly in Finland.
Around the year AD 200, speakers of North Germanic became distinguishable from other Germanic language speakers. The earliest evidence of this separation comes from runic inscriptions.
Written Icelandic is closest to Old Norse among the modern Scandinavian languages. Old Icelandic was essentially identical to Old Norwegian until at least around 1000.
Mutual intelligibility between the three Continental Scandinavian languages is asymmetrical. Norwegian speakers score highest at understanding the other two, averaging 6.85 out of 10 in a 2002-2005 study. Swedish speakers in Stockholm averaged only 3.46 when tested on Danish, the lowest single-language score in the study.
Bokmål developed from Danish influence during the Denmark-Norway union of 1536-1814, while Nynorsk was constructed from western Norwegian dialects after Norway became independent in 1814. The two norms reflect the country's divided linguistic history and remain both officially recognized.
Elfdalian, spoken in the Älvdalen locality of Dalarna, is considered a separate language by many linguists because it lacks mutual intelligibility with Swedish. It has an official orthography and shares more features with West Scandinavian dialects than with standard Swedish.