So, the dna had to be pure, no Nephilim lineage. So, if the land of Caanan was filled with Nephilim, and then the great grandmother of Jesus hailed from this group, explain.
So, giants were settled in the land that God promised the Israelites. We have to remember that the Israelites were slaves in Egypt for many generations. They were basically isolated from the rest of the world from the death of Joseph to the birth of Moses, giving the Nephilim and the rest of the -im tribes plenty of time to build up fortresses in Israel.
And, all of Israel didn't have to be completely pure of angel DNA. Just Jesus' lineage. It's unlikely the angels (at least, after the flood) would be able to again corrupt all of mankind.
Something different happened when the second wave of watchers fell. It seems their strategy changed.
God created the whole earth in six days. He has an infinity of angels available to Him. With everything He's done, you think He'd mess up the tiniest detail in his son's human lineage?!!!!
Start reading the beginning of the New Testament. You must not realize that our God is the perfect One, and WE are the ones that make mistakes.