On the Petros/Petra difference. Yes, you were taught a fairly common exegesis of that passage which attempts to distinguish between the meanings of petros and petra. I am by no means a Greek expert, but I do not think that distinction applied in Greek of the first century, though it *may* have earlier in Attic Greek. I will look it up in my Liddell-Scott lexicon when I get home, and I'm pinging jrny whose Greek is better than mine.
But even if that were so, that distinction does not seem to be what Christ was intending here. You mention the Aramaic Gospel of Matthew. We have evidence from the 2nd century that states that Matthew wrote in Aramaic (or Hebrew), but obviously that kind of statement is not infallible. What is infallible, however, is the Bible itself, which repeatedly calls Peter Cephas--the Aramaic form. Furthermore, John 1:42 shows which was the original form when it says "Cephas which, translated, is Peter." If Christ originally used Greek, what the Bible would have said was the reverse.
There is an easy explanation, moreover, why there are two different forms of "petr-" here. Matthew wanted to imitate the Aramaic in Greek: so he wanted to play on the word "Rock". However, in doing so, he was constrained to change the gender of feminine "petra" to make it fit a man: "Petros".
This is the explanation that best fits the available evidence and does not twist an interpretation out of the text which is not there.
I'll have to look into my Lexicon as well, which defines varying forms of Greek from Homeric to Attic to New Testament time periods.
"so he wanted to play on the word "Rock". However, in doing so, he was constrained to change the gender of feminine "petra" to make it fit a man: "Petros".?"
I agree. And besides, the etymology of the word means that Peter means the same thing as rock, the difference in gender being a grammatical necessity.
"There is an easy explanation, moreover, why there are two different forms of "petr-" here. Matthew wanted to imitate the Aramaic in Greek: so he wanted to play on the word "Rock". However, in doing so, he was constrained to change the gender of feminine "petra" to make it fit a man: "Petros"."
You are right on the money, Claud. By the way, "Petros" as a name probably translates as "Rocky" Its really a triple play on words but in Greek.