Were the Watchers referred to in the noncanonical Book of Enoch actually the Anunnaki extraterrestrials present on the Earth, and the Nephilim actually hybrids of Anunnaki/human mating, as is true of the Nephilim described in the Bible?

These are both true. The Watchers was the term used, represented the lofty ones, the gods from the sky who mated with human to create the Nephilim and this too was a race of giants, although not as gigantic in stature as the pure Anunnaki, were giants nonetheless compared to full-blood humans.