Further, we have the evidence provided by the Nomina Sacra, which practice owed nothing to the Jews' tetragram.
What makes you say that? From what I have read, most scholars do see a connection between the treatment of the Tetragrammaton in Jewish texts and nomina sacra forms in Christian texts. The exact relation and development are much disputed, but most scholars posit some relationship: Traube, Paap, Brown, Roberts. Comfort, even Hurtado accepts there's a connection. See this article on the subject which contains an overview, as well as a proposal: