Short answers, pulled from the story.
Old Norse began to fracture across Scandinavia in the 8th century when Proto-Norse split into distinct branches. By the Viking Age, this language had divided into two major branches known as Old West Norse and Old East Norse.
Old West Norse spread from Norway to Iceland, Greenland, the Faroe Islands, Ireland, Scotland, the Isle of Man, and parts of northwest England like Cumbria. It also reached Normandy through Danish settlements there.
By the 11th century, Old Norse stretched from Vinland in the west to the Volga River in the east. In Kievan Rus', it survived longest in Veliky Novgorod, likely persisting until the 13th century.
Standardized orthography marked long vowels with an acute accent, though medieval manuscripts often left them unmarked or used gemination instead. The standardized Old Norse spelling system was created in the 19th century and is mostly phonemic today.
Runes continued in common use until the 15th century and appear to have persisted in some Swedish regions as late as the 19th century. When Christianity arrived in the 11th century, the Latin alphabet replaced runes for most purposes.