Relationships between God the Father, Mary and the two natures of Jesus Christ Would it be okay to say this:


  Jesus Christ is both God and Man, which means that He possesses both
  divine and human nature. Divine nature in Him is from God the Father
  and the human nature in Him is from the virgin Mary.


or what I said just now - I am mostly concerned about the very last sentence - would be a heresy according to the teaching in the main-stream Christianity?

If the last sentence in the quote is a heresy, please, specify, which branch of Christianity considers that as heresy.
I'd imagine that would be a heresy of the heresy of modalism or arianism or something, but that's not a very precise answer since a heresy of a heresy could be anything.

So, if you want to identify a heresy to the doctrine of the incarnation it would have to deny that Mary is the Holy Mother of God.  The heresy that denied that was Nestorianism. You can read St. Cyril give him the business in his 12 Anathemas.

The miracle of the Incarnation of Jesus took Mary's flesh and made Jesus, like God took part of Adam and made Eve, except in reverse; this is all very fitting and proper and a completion of the work of the OT, as it should be. Moreover, it is a mystery and a miracle that cannot be understood by reason alone, as it should be.