This is not nitpicking, it just means your observations are obvious because they rest on all kinds of non-conscious, tacit, consensual knowledge beyond that.

That's what nitpicking is!

And that is roughly what Quine meant: I can prove hardly any time train ever departs on time if I just make another change in the system, such as saying on time means nanosecond exactness.

Changing the definition of the words in a sentence does not change the proposition that was originally expressed by the sentence. It just creates a different proposition expressed with the same words, and is irrelevant to understanding the original one.