Yup, the dative and the ablative were often the same, as well as the nominative and the vocative, and the locative was almost dead in the paradigm of 1st c. BC Latin already. It's not that it suddenly became disfunctional, but just that such semantic relations started to depend more and more on prepositions in more contexts, besides phonetic mergers that increased the number of syncretisms among the cases.
That means Latin has been a dead language for a very long time. Well, I guess it's kind of like French.
What? Then what did you call the language spoken by Romans in the 1st c. BC, or the language of the French today? They're not dead, they just have different registers and a writing system that progressively gets farther away from a 1:1 relation with the phonemes. In English <meat> and <meet> are pronounced the same (or at least the great majority pronounce them the same), but that doesn't mean English is a dead language, as in, a language that's not used in speech...
This also supports the idea of sound changes taking a long time as opposed to the supposed rapid evolution proven possible by the Romance languages...though maybe a lot has to do with accents appearing from underlying displaced languages. What's the theory on that? Rome conquering people's and things happening because of it?
People innovating pronunciations and the expansion of these afterwards. See William Wang's often quoted article using Chinese data on lexical diffusion: "Competing Changes as a Cause of Residue" (1969), and much recent research on free variation (see for example the many
allophones in free variation in Cantonese described in Bauer and Benedict's Modern Cantonese Phonology
, various of which are even attested in descriptions of Cantonese phonology from 150 or 100 years ago).
I'm pretty ignorant of the evolution of Catholic rites, what are you referring to here?
^^The church deliverd sermons in Latin up into the 8th centuries, supposedly because people stopped being able to understand them, and began speaking in the vernacular.
Well... here's when we get into what exactly people weren't able to understand, and how written Latin was read.