Isn't Israel the firstborn Son of the Lord? We read in Exodus 4:22 that the people Israel are God's firstborn son:


  "Then you shall say to Pharaoh, 'Thus says the LORD, "Israel is My
  son, My firstborn. (NASB)


So now, how can Jesus be the firstborn Son of God, let alone the only Son?

Several of the "prophecies" about Jesus mentioned in the Gospels refer to what is in fact a statement about Israel the past and present. For example Matthew describes the family of Jesus leaving Egypt and says:


  "He remained there until the death of Herod. This was to fulfill what
  had been spoken by the Lord through the prophet: "OUT OF EGYPT I
  CALLED MY SON."" - Matthew 2:15 (NASB)


The Prophet referred to in Mathew 2:15 is Hosea, who says:


  When Israel was a youth I loved him, and out of Egypt I called My son.
  2 The more they called them, the more they went from them; they kept
  sacrificing to the Baals and burning incense to idols." - Hosea 11:1–2
  (NASB)


The problem I have with such interpretations of Jesus' actions as fulfilments of propecy are that they were 1) about Israel 2) statements about the past rather than predictions and 3) the behavior being described is hardly matching the character and actions of Jesus in the gospels.

How do Christians reconcile the logic of Jesus being God's firstborn son or even the only son, with the actual words of God in the Old Testament about the nation of Israel? I don't mean metaphorically, but what is the actual reality? How are the later verses in the Gospels interpreted? On the one hand they seem as "additional" interpretations of the "original" verses about Israel, but read "in the light of the Gospel". But that can't negate the original meaning, can it? In reality, which is it?
Abraham's first-born son was Ishmael; but God referred to Isaac as Abraham's only son in Genesis 22:12:

Now I know that you fear God, because you have not withheld from me
your son, your only son.

Abraham sent his first-born son, Ishmael, away, making Isaac his only son - his only legitimate son, just as God sent his first-born son, Israel, away (after Israel sinned and sinned and sinned again), making the Lord Jesus Christ his one and only son - his one and only legitimate son, the Messiah.
Just as Isaac was the fulfillment of God's promise to Abraham, the Lord Jesus is the fulfillment of his covenant - with Abraham and with all of humanity. And we know that Isaac is symbolic of Jesus as a sacrifice, with Abraham's righteous willingness to sacrifice his own son as God did for us.
There is also a recurring theme of the second-born claiming the birthright that the firstborn rejects, or displacing the firstborn - see Esau and Jacob, Perez and Zerah, and later Jacob putting Ephraim ahead of Manasseh. To some extent this applies to Cain and Abel - the firstborn sins and is exiled, and the second-born does what is right.