>>9 Depends on how much you can verify how accurate a historic language is. I guess the cut is where it stopps being useful to interpret the world.
The only time I use old languages is to understand new words (eg "define word" in google then it gives you the cool tree of origin) or read old texts. If there exists text and the we know somewhat accurately what the symbols and words mean the lang probably is useful.
An interesting example would be "In the beginning was the Word", the first line of the bible. Doesn't make much sense in English but if you know that the pre-translated word was logos not lexis and know what it meant in Greek at the time it makes sense. When I first heard that a new world opened.
No idea if my understanding is correct but I found out that old civilisations had less separation from word and thought. The Egyptians didn't write down the bad parts of their methology because they believed it had power. So if you're interested in that you've to search 3rd party sources. Or the Greeks thought of logos as kinda the flow of the world. I'd have to invest more energy to understand, I think the concept isn't really around anymore so one has to describe it instead of translate. Of course, nowadays, depending on the ideology, people interpret logos differently.