The passage Jer. 31:27 God returning life to Israel and Judah does not say the genetic traits are mixed. If I said I was sowing carrots and corn in my garden you would not expect to have a cross between the two but individual rows of distinct separate plants.
The scripture doesn't say that.
"Behold, the days come," saith the LORD, "that I will sow the house of Israel and the house of Judah with the seed of man and with the seed of beast." (Jer. 31:27)
It refers to the House of Israel and House of Judah which is a common reference to the people of Israel and Judah, which means that the see of man and the seed of beast are sown into the people. Actually, we already see the 'seed' of the mechanical beast being sown into people (e.g., pacemakers, etc). Soon people will be made of all kinds of mechanical and biological components from other animals.
In regards to Seth, yes God did give us the ability to create life, but with it he also gave strict boundaries. I am of course referring to marriage.
I think you are missing out on the comparison of the language. Seth was made in Adam's likeness and image, as was Adam made in God's likeness and image. To be made in the likeness and image is more than being born from them. The Hebrew word for image is tselem and it comes from an unused root meaning 'shadow'. Humanity is the shadow of the Creator which means our duty is to create. This is even implied in Gen. 1:26 where God said let humanity have dominion over the creatures. Notice God did not give man dominion over the earth - the dominion was specific to the animal kingdom. In Hebrew the word for dominion is radah and it literally means to rule as the Christ will rule over humanity. Here are two scriptures specifically referring to the Son of God where radah is specifically used:
Psalm 72:8 "He shall have dominion [radah] also from sea to sea, and from the river unto the ends of the earth"
Psalm 110:1-2 "The LORD said unto my Lord, Sit thou at my right hand, until I make thine enemies thy footstool. The LORD shall send the rod of thy strength out of Zion: rule [radah] thou in the midst of thine enemies."
In other words, as the Son of God is to reign over the universe forever and ever, so humanity is to rule over the creation. The Son of God has the distinguished honor of being the creator of life (John 1:3), so it only makes sense that humanity having 'dominion' (having 'radah') over the animal kingdom is to be granted the power of creation. Jeremiah 31:27 makes sense.
I might add, though, that this power is an awesome responsibility, one in which needs the wisdom of God to implement. We are too early in our evolutionary development to move aggressive in this direction, but it is our destiny Luke.