1.- The Nephalim, the Watchers, and the Giants

<---True! The Nephalim were the offspring of human females and Angels!
The offspring were abnormally large! Perished in the great flood of Noah's day.

2.-- What Jesus meant in Mat. 24 by "marrying and giving in marriage"

<---Going about their normal lives. People today marry and give in marriage.

3.-- The REAL reason God had Israel wipe entire nations off the map, including children and animals

<======Other nations
of those days were usually warring with the Ancient Jews, the other nations were filled with disease from having sex with animals, their offspring, their cousins, their mothers and fathers and were full of disease. Even the animals were diseased from being used for sex.
There were no hospitals in those days remember?

People don't realize that the world in ancient times had no laws or rules of conduct. That is why God used the ancient Jews to teach the world that humans need rules and laws.

God cleaned up the ancient Jews as an example for the world at that time to see.