And in that respect your interpretation of Isa 51 parallels the non-Catholic Christian interpretation of Matthew 16 - i.e. that Peter is the first to be hewn from God (The Rock) having been the first to receive the revelation from the Father that Jesus Christ is Lord.
Yes...my view may be a little different that is the norm. I can see the petra-petros difference, but not attempting to look at it in the "pebble" versus "rock" terms that so many Proddies use to be intentionally disrespectful of St. Peter. However, I also can appreciate the Aramaic form, where "kipa" is used in both instances. If you take a look at the traditional history of the Gospel of St. Matthew (as captured in Eusebius' Church History), you will note that this work was originally written in Hebrew and then later translated into Greek. Of course, no Hebrew version of Matthew exists; but the Aramaic DOES exist...in what became the Peshitta. Bottom line is that either way of looking at it doesn't destroy my theology one way or the other.
More critical is my view of the keys (Mt 16:19). That is rather unambiguous and the view is formed via Isa 22:22 and other OT texts relating to Eliakim (son of Hilkiah). I do not state that Peter is the King. Jesus is the King. I see Peter's role is that of the King's steward...his "prime minister," if you will.
That's why (for me), the issue of petra vs petros or kipa vs kipa really doesn't matter all that much. In both instances, we know who the King is and who the minister is.
And, btw, you are spot on about Abraham. That's why the use of the word 'rock' may be simply an allusion.