Hi all, and welcome to @Alex-Wynne. Nice to see you here! I enjoy your works. What I’ve read, the ideas and arguments are always very clear.
I’ll see if I have the time (and skill!) to read this discussion and quoted articles in detail later. I just skimmed over it all, and it all seems a little out of my wheelhouse. In my essay which Dogen just linked, I take quite a different approach as well.
So to potentially save people some time, in context of this discussion, my thinking may be summed up as follows:
Venerable Nāgārjuna indeed quotes a version of the Kaccānagotta Sutta: this is well-known. But that alone doesn’t make the text proto-Madhyamaka, of course. I (Sunyo) could quote any sutta and reinterpret it in my own idiosyncratic way—either consciously or accidentally—yet that wouldn’t make the text “proto-Sunyo-vāda” (pun intended!)
And nobody would argue in this way. But I think the question we should answer first is: did Nāgārjuna actually interpret the Kaccānagotta Sutta correctly? That’s what I try to do in the essay. And I conclude, “no”. No typical (proto)Madhyamaka or Prajñāpāramitā concepts are actually being discussed in this sutta. That is a later reinterpretation (either consciously or accidentally).
Based on various early sources I conclude that the two wrong notions in the sutta don’t refer to abstract notions of existence and nonexistence but specifically to notions of the afterlife. I find the most interesting source the Sanskrit Mūlasarvāstivāda Vinaya, where nāstitā (i.e. natthitā) describes the materialist view that there is no life after death.
I plan to write more on this in the future. For now, I personally failed to find any typical Madhyamaka (or proto-Madhyamaka, whatever that is) ideas in the Pali canon. In the Pali canon, the middle view always avoids eternalism and annihilationism, being notions of the afterlife, not abstract notions of existence.
In contrast, a text that does have a proto-Madhyamaka flavor to me is EA15.2.
I translated ayasmā kaccānagotto, perhaps a little liberally, as “a venerable one from the Kaccāna clan”.
To support your argument from lineage, perhaps it’s worth considering other name-gotta’s as well. Take Vacchagotta. Was he a single person or was the Vaccha clan turned into some kind of archetype in the suttas? I also translate vacchagotto paribbājako as ‘a wanderer from the Vaccha clan’; see my footnote 33 here (clickable).