Short answers, pulled from the story.
Proto-Norse was an Indo-European language spoken in Scandinavia from around the 2nd to the 8th centuries CE, corresponding to the late Roman Iron Age and the Germanic Iron Age. It is considered the earliest stage of a distinctly North Germanic language and evolved into Old Norse at the beginning of the Viking Age around 800 CE.
Proto-Norse is known entirely through Elder Futhark runic inscriptions carved into stone, bone, and metal. About 260 such inscriptions survive, with the earliest dating to the 2nd century CE. Examples include the Øvre Stabu spearhead from Oppland, Norway, and the Tune stone from Østfold, Norway, dated to around 400 CE.
Proto-Norse evolved into Old Norse dialects around 800 CE, which in turn became the modern North Germanic languages: Norwegian, Swedish, Danish, Faroese, and Icelandic, along with their dialects.
The Golden Horn of Gallehus 2, found in South Jutland, Denmark, and dated to around 400 CE, carries the inscription ek hlewagastiz holtijaz horna tawido, meaning "I, Hlewagastis of Holt, made the horn." It is one of the most famous Proto-Norse texts and shows early evidence of a-umlaut.
Yes, numerous Proto-Norse loanwords survive in Finnic languages including Finnish and Estonian. Finnish kuningas (king) descends from Proto-Norse kuningaz; Finnish runo (poem, rune) comes from a Proto-Norse word meaning secret or mystery; and Finnish juusto (cheese) traces back to a Proto-Norse form related to Old Norse ostr. A large Proto-Norse loanword layer also exists in the Sámi languages.
Between 500 and 800 CE, two major processes transformed Proto-Norse into Old Norse: umlaut, in which vowels shifted under the influence of following vowels or semivowels, and vowel breaking, in which single vowels developed into diphthongs. A third process, syncope, eliminated most unstressed vowels, converting words like katilōz into katlar (cauldrons) and habukaz into haukr (hawk).