OK, my memory did not serve me. I'm looking at the Greek behind the text on [BLB](https://blb.sc/002cNF). It was **not** two completely different words (my apologies), but two variations of the same word: "Petros" (masculine, "rock," referring to Peter) and "petra" (feminine, "cliff" or "boulder" - referring to himself, or to the gospel message itself). I do notice, however, in support of your argument, that all the "thees" and "thous" in 16-19...are in the singular form. That's worth thinking about...

I think your second paragraph is spot on, and that's exactly the issue.