Nice work. You make some good points and make some guesses based on logic and supporting evidence that I agree with.
Hurray! That is exactly what I was going for when I was writing this, so I'm glad at least one person thinks I achieved that.
However, I'd like to add something of my own. Regarding who was the Sith Lord that struck a deal with Sel-Makor, I believe that it actually was Baras.
I you check the Q&A section(which is more of a misc and random musing section than anything else), I have actually started keaning towards Baras since I started working on the guide. It just sort of hit me while I was working on it. I am inclined to agree with you that Baras likely was the geat Sith Lord. I do have some reservations of considering that stance to be definitive or well supported though because there are just a lot of open questions if Baras was the great Sith Lord such as what did Sel-Makor give him and how/why did he get to Voss 20+ years ago, presumably before Voss was officialy discovered.
He did in the book Revan and we can pretty safely assume that it was the actual body since it was 300 years before the Treaty so the Emperor didn't depart yet. Also, when Scourge first met the Emperor in the Citadel, the Emperor showed him his mind for a split second which resulted in Scourge collapsing on the floor, crying like a baby and it involved Scourge looking directly in Emperor's eyes.
As I said, my position on that specific point is going to be controversial. I admit it is not a particularly well supported position. However to counter the points that you raise, according to the codex the Emperor has been using the Emperor's Voice for centuries to deliver "the Sith leader's commandments to his servants." That means the Voice has been in use since long before the Treaty of Coruscant, and has potentially been around since before Scourge became the Emperor's Wrath(assuming that centuries doesn't mean just 200 years).
As for Scourge being shown the Emperor's mind for a split second, the Voice has the Emperor's "power and consciousness" according to the codex. That means that the Voice would be able to show Scourge the Emperor's mind just as easily as the Emperor's True Body would be able to.
That said, it's very ambiguous as to whether the "Emperor" in JK chapter 2 finale is Voice or the actual Emperor's body since on one hand we know that Emperor left known galaxy after the Treaty and on the other hand we have a Voice in Voss body and by all accounts, there can't be 2 Voices at the same time. My personal belief is that Scourge doesn't know about Emperor's departure and that it was the Voice JK saw in chapter 2 but masked so that Voss face isn't visible. However I don't have any solid proof for that so don't quote me on that.
I agree with your stance here, and I like to think my reasoning for this is explained sufficiently in the various sections of the Primer.