I have the official translation LN here and looked trough Volume 7 and the only time there is talk about the Demon Peer is when Diablo talks to Battlesage/Great Saint Saare and there he only says that he has completed his evolution to an Demon Peer, but without any indication that he already was one when he was summoned.
I cant talk about the furigana ect stuff, i have to trust the official translation in that, as i am not able to speak or read any japanese at all, but i dont know why you get over that in such detail anyway, since we agree on that point, yes, Diablo could have become an Demon Peer himself, as you explained, trough holding his magicules at maximum for a long time and as said, the Demon Peer is, while maybe in power similar, something inherently different from the "real demon lord" rimuru elvoved to. On the other hand "Demon Duke" and stuff is just a title, that indicates that that one is special/stronger than normal, but not some sort of evolution in itself. So, no disagreement there
What we have is the differing opinion about Diablo being Arch Demon or Demon Peer when summoned and i couldnt find any clue in Volume 7 that said he evolved before he was named, only that he evolved, which is obvious at that point.
However, in Volume 6, when Rimuru mets one of Guys follower Maids, which are Demon Peers, he ponders about her strength and the way to trigger the evolution (since it is "special" and not only a "enough energy" thing), saying that it was him for diablo who triggered it, and in Volume 5, when Rimuru names him Diablo, Raphael reports "The individual Diablo was an Arch Demon" when Rimuru wonders about the used Energy, while DIablo himself gets a cocoon around him, preparing for his evolution (plus the use of "was" would point to an ongoing evolution right now?)
So, right now i dont see any indication that he evolved before he was named, on the contrary, it would be weird that Raphael called him an Arch Demon before naming, since he should be the one who knows the best aside from diablo himself.
Extra Point about the "families", is ever mentioned that the progenitors "rule" over the families? I do remember that they share certain characteristics (like, that the black ones are all... a bit weird, like diablo, differing from the normal "power hungry" demons, but its not as if Guy Crimson has every Demon of the Red, Green and Blue Families under control right?