You also have to remember the context in which the “chants” were passed down. It is better to see the Rig Vedas and the Vedas as a ritual manual for doing a successful rite. And the success of the rite was paramount. One wrong action, one wrong word would spell disaster for the efficacy of the whole endeavor. People prepared for a long time to do the ritual.

You had several priestly functions, like the Hotar, who recites the invocations of the Veda, and there is also the Brahman. The Brahman checks if everything is done to precision and no mistakes are made. If there are, they need to do corrections.

This is another reason why one can say it has been passed down without much change. There is a critical edition by scholars that reconstructs the changes in meter that might have occurred, but nothing else.

No, all of the texts we're talking about here have been passed down in written form for 2000–3000 years, if we ignore the Scandinavian ones. It was only the first 1000–1500 years of the preservation of the Rig Veda that were exclusively oral.

The Rig Veda was written down thousands of years ago.