I see your point of view but the fact is that Eternity is specifically identified as being the seventh and eighth multiverses instead of the seventh and eighth iterations of reality. To me, that really reinforces the idea that there have been a total of eight multiverses, not one universe and seven multiverses.
It's 1 universe and 7 multiverses with a total of 8 iterations. Living Tribunal says in issue 2 that he's the living tribunal of the 8th cosmos (iteration). There wasn't a 9th iteration.
The problem stems from the fact that writer Al Ewing has sometimes used the terms "cosmos" and "infinity" and "multiverse" and "omniverse" as if they were interchangeable words that all meant "iteration of reality." However, that is NOT the case. For one thing, THE Omniverse is made up of ALL realities, including all of the various Multiverses and any other individual universes that exist. So, all references to Eternity being an Omniverse or to Galactus being from the Sixth Omniverse are totally and absolutely WRONG!!!
Next is the term "cosmos" which Ewing has often used it as a synonym for "multiverse." In New Avengers Vol. 4 #2, the wizard Mor-I-Dun identifies himself as being "of the Fifth Cosmos" and the current post-Secret Wars reality as being "the eighth." This idea is supported in Ultimates 2 #2 when the Living Tribunal identifies himself as being "of the Eighth Cosmos." And then there's the fact that Eternity's death and resurrection represented his change from being the seventh multiverse into being the eighth multiverse. Taking all these statements together, one could see the current Eternity as being both the eighth multiverse and the eighth cosmos. However, all of these statements were made before the existence of the original universe was revealed.
Once the First Firmament is added to the equation, things get messy in two different ways. First, in Ultimates 2 #6, the First Firmament makes it clear that a cosmos and a multiverse are NOT the same when it identified the first of the multiverses as being the "Second Cosmos." So, from this point on, "cosmos" could be a synonym for "iteration of reality" but not for "multiverse."
Second, at the end of Ultimates 2 #5, the First Firmament makes a very clear distinction between himself (one cosmos/one universe/one reality/one totality) and Eternity's "foul kind" (the Multiverses, of which there are eight). So, since the First Firmament definitely does NOT count himself as a multiverse, why should you?
At this point, your concept of "1 universe and 7 multiverses (for) a total of 8 iterations" no longer works. A universe and a multiverse are both iterations of reality and they might both be called a "cosmos" (plural cosmoses) but they are NOT the same thing. It would be like saying "One sparrow and seven swans make a total of eight birds because every bird is also a swan." That's just not correct.
So, since Eternity is the eighth MULTIVERSE and a multiverse is NOT the same as either a cosmos or a universe, that means there have actually been nine iterations of reality/cosmoses, and any entity who identified realities of Mor-I-Dun, and Galan and the current one as, respectively, the Fifth, Sixth and Eight Cosmoses must have been wrong (or simply unaware that the First Firmament's original universe ever existed).
The only way the situation in issue #100 works is if everybody who referred to the current reality as the Eighth Cosmos knew of the existence of the original universe and if the First Firmament was wrong when he claimed that Eternity was the seventh and eighth multiverses. I don't see any indications of either of these conditions being true.
The cosmic beings may not know every detail but they knew they were in universe 8. So they knew there was at one point a universe 1. They may not have known it still existed outside of the current multiverse in a void plotting revenge but they knew there were 7 previous universes. They've said it constantly. Often characters say universe or cosmos rather than Multiverse.
And the splash page with the Eternities is all the proof I need to know I'm right. That told us there were 7 iterations of Eetrnity including the First Firmament and the current Eternity representing iteration seven and eight.