19. Is everyone descended from Adam and Eve?

Yes, everyone is descended from Adam and Eve.

And Adam called his wife's name Eve; because she was the mother of all living. Genesis 3:20

No, some are not.

Melchisidec just appeared, without father or mother. So he wasn't descended from Adam and Eve.
Melchisedec, king of Salem, priest of the most high God, was Without father, without mother, without descent, having neither beginning of days, nor end of life; but made like unto the Son of God; abideth a priest continually. Hebrews 7:3
The Nephilim were the giant offspring of the sons of God (angels?) and the daughter of men. So they weren't desscended entirely from Adam and Eve.
The sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. ... There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown. Genesis 6:2-4
And at least some of the Nephilim survived the flood, at least according to the book of Numbers. (See the NIV translation below.) So maybe there are still some giant offspring of God's sons around today!
And there we saw the giants, the sons of Anak, which come of the giants: and we were in our own sight as grasshoppers, and so we were in their sight. Numbers 13:33

We saw the Nephilim there (the descendants of Anak come from the Nephilim). We seemed like grasshoppers in our own eyes, and we looked the same to them." New International Version

Christian Responses
Liberty Gospel Tracts
Uncommon Descent