Short answers, pulled from the story.
French linguist François-Just-Marie Raynouard proposed that Romance languages descended from a single ancestor distinct from Classical Latin. He called this ancestor la langue romane and believed it replaced Latin before the year 1000.
The genitive case died out around the 3rd century AD according to Meyer-Lübke. It began to be replaced by de plus noun constructions as early as the 2nd century BC.
Scholars reconstruct features of non-literary Latin through specific types of surviving texts including Pompeian graffiti, curse tablets, private letters, business records found among papyri from Egypt, and tablets discovered at Hadrian's Wall. Technical works like the Mulomedicina Chironis veterinary treatise and Christian texts also provide data on colloquial terms and pronunciation habits.
Word-final /m/ disappeared in polysyllabic words by the end of the empire while monosyllables retained it as /n/ before fricatives caused compensatory lengthening of preceding vowels. Front vowels in hiatus became [j] which palatalized preceding consonants over time.
Vulgar Latin shifted from synthetic to analytic structures due to untenable case systems after phonetic mergers. Prepositions increased in number to perform syntactic functions formerly served by inflections.