Or, to put it another way: If the Church hadn't preserved Latin, would it even be considered a different language from Italian as opposed to simply an older form in the development of the Italian language?
Pondering over [question not very related] I happened on still live (painting), inanimate, that in some language translates as "dead" (cp perhaps dead weight). Latin does not change as its grammar fossilized, deemed perfect by some, irregardless of new nouns being added.
The second part of your question has been answered before at what point does a language become its descendent. Spoiler: when (mutual?) interligability is lost; that's necessarily a fuzzy concept.