Using the term crewed instead of manned is more inclusive

While we don’t know for sure, water probably makes up something like 0.05% of earths mass, which is not that much at all! And in fact, you can easily acquire this much water by only accreting the driest types of planetary building materials (eg enstatite chondrites) without needing any contribution from the water-rich bodies discussed in this article. Though, these water-rich bodies almost certainly did deliver some fraction of earths volatiles!

Most of the oxygen on earth is locked away in the rocks and minerals that make up the earth itself! The free oxygen in the earths atmosphere is almost entirely created by life. The first life forms on earth were not aerobic and didn’t use oxygen to metabolize.

Keep in mind that once earth became sufficiently large, even volatiles that were vaporized would still be retained. Just think about how we can boil water today and it’s not like the water vapor disappears to space.

Earth formed by the accretion of countless small bodies ranging in size from pebbles to Nara-sized bodies. Some fraction of these planetesimals formed in the region where earth exists today (inner solar system) and likely had enough water to meet earths current inventory. Another probably smaller fraction of bodies from the outer solar system (like Ryugu and the other progenitors of CI chondrites) were also accreted and surely delivered some fraction to water. But to answer your question, earth was accreting water-bearing asteroids and planetesimals all throughout its accretionary history. It wasn’t a one-off event or even just a few random chance impacts.

Life appears to be an essential ingredient for life so a planets ability to accrete and retain water is critical. But despite the assertion in this article, earth likely accreted with enough water that did not necessitate the need for extraterrestrial delivery. Though, it is highly likely that these highly volatile bodies did deliver some fraction of earths water.

Yes. We measure water in meteorites that have survived atmospheric entry. A tremendous amount of mass (including water) is lost (vaporized) during entry but most of it should be retained in the atmosphere.