I don't think that Grant Morrison's interview statement slightly implying that the Presence, the Overvoid, and the Source are the same being is enough evidence to say they are. He actually doesn't even say they are, he says different beings in DC have different things that they believe to be akin to a monotheistic God, the Monitors believing the Overvoid, the New Gods believing the Source, and the humans believing the Presence. They're not only typically portrayed as different beings, as well as the evidence for them being weak [I.e., the one of the Over-Monitor saying the Source is the "Prime Monitor". Monitor-Mind the Overvoid has never been referred to as the Prime Monitor, only the Primal Monitor, which Prime and Primal mean two very different things. There is also the comic where the Spectre goes into the Source and gives the implication that the Source is God, but that is moreover because the Spectre's narrative at the time is that he is travelling pantheons to learn the nature of God, and this was the New Gods' interpretation of God]. They also consistently are portrayed as having different levels of power. The Source is a very down to Earth source of power, being fully harnessed by beings such as those who wield the Quintessence, Mister Miracle in Death of the New Gods, Highfather becoming the Source after dying, and the several times Darkseid threatened the Source and even fought it. There is Mageddon, who came from beyond the Source, the true form of Brainiac who went beyond the Source, the examples are endless. The Presence is always superior in stories, often portrayed as even being able to hold the Source in his hand. Beings who deal with it and threaten it are never on his level, really. And this extends even further with the Overvoid. The Overvoid and the Source are even explicitly separated on the Map of the Multiverse with the Source being contained within the Overvoid. Having these three as one being would also accumulate a massive amount of anti-feats that the Source and the Presence have and impose them onto the Overvoid.
As for the metafictional stuff for DC, I don't think it needs to be entirely gotten rid of, because DC does have characters that are intentionally that meta, and reoccurringly so. I don't think the Writer should necessarily be as powerful as it is, though. The Writers in DC we see in the series are variable in power. There is the Monkey and Grant Morrison, who both lived in Limbo and are portrayed as being nowhere near as powerful as the Overvoid, for example, but then we have the Writers of Retconn Corporations from the Milk Wars comics that are shown to be superior to the Overvoid, but also inferior to several other serious and non-meta characters, such as Ahl and the Eonymous, the latter likely being the strongest beings in DC, as the Writers of Retconn only make stories on the Overvoid to appease the Eonymous to keep them from destroying everything.
Anywho, it can be talked about more in-depth in another thread, I suppose. I saw the proposition and thought I would make a comment.
