Of course the big hole in your argument is that there are 2 different words in the original Greek. You can't get around it. And spare us the "Matthew was not written in Greek" baloney, unless you can produce the Aramaic manuscripts from that time.
Gladly (but we never show them to Protestants :-))
I don't need to try and convince you that the original text was in Aramaic - that's irrelevant. But everyone agrees that Christ certainly said it in Aramaic.
Oh but if we could only know what He said in Aramaic we could solve this puzzle (whoa is me).
But wait! We DO know what he said in Aramaic because we know that Peter's name (in Aramaic) was Cephas (kay-fas), which means - you guessed it - "foundation stone".
So why the apparent confusion between Petra and Petros? Pretty simple actually - One is the feminine form of the noun and can be used for an inanimate object, but never for a masculine name - the other is the masculine form an can be used, even if it also implies a different size rock to some.
I wasnt aware that Jesus spoke in Greek to his disciples.