If you read Tertullian's arguments in the beginning of Adversus Marcionem carefully, he defines monotheism as one divine substance.
This is getting laughable. You are quickly becoming a parody of an amateur scholar. Tertullian never once mentions 'monotheism.' I am not even sure if such a technical term ever arose in the first ten centuries of Christianity and if it did it was exceedingly rare. So - yes, I've read the entire book carefully (not just the beginning) and - no there is no mention of monotheism.
There could be a billion persons in that substance
But there wasn't so what's your point.
and to Tertullian it would still be one god.
Yes there is a funny David Allen skit about this very subject. Three is not one. It's not important what Tertullian or anyone else says on the subject - it's an ancient precursor to Trump's 'fake news.' Three is not one. Even Siamese twins are still two individuals not one.
What makes two gods is the assertion of two separate divine substances.
Yes that's exactly my point - something you apparently can't see or understand. ANY SECT which argues that the Father and the Son are separate would necessarily have postulated (in the manner of the criticism here) that one was 'alien' or 'another' to the other. The point is that the terminology used against Marcion might simply be a vestige of this sort of a critique (i.e. used by Patripassians against those who didn't 'fully' acknowledge one substance shared between Father and Son). It's all a matter of perspective.
Since Tertullian well understand this theory, it makes no sense to suggest the Marcionites are being misunderstood on this point
This is the stupidest thing you've said in the entire post. Tertullian isn't postulating a 'theory' for fuck's sake. He speaks throughout of a 'rule.' Tertullian's source for this text Against Praxeas is clearly Irenaeus who uses the same terminology. The point is that it is not a 'theory' or something which someone can have actual expertise (unless one is commenting on Tertullian's or Irenaeus's habit of making such wild claims). Tertullian simply raises his hands and says 'it's true.' And of course it is worth examining with respect to Marcion because the same sources (Irenaeus and Tertullian) raise there hands there and say 'it's true' too.
Tertullian and Irenaeus aren't 'scholars' or 'academics.' As a matter of fact they remind me a lot of you - they just make assertions and assume that everyone has to go along with what they say because it makes sense to them. It is nonetheless very curious how close the criticism of Marcion and the criticism of anyone who actually gives character to the obvious limits between the 'one nature' shared by Father and Son.
The Arians for instance agreed that there was one nature UP TO A POINT. The Son was still somehow 'less' than the Father. He was an instrument of the Father etc. If 'justice' and 'mercy' are distinct then each power in the Marcionite godhead has 'otherness' in relation to the other. There isn't absolute unity NOR CAN THERE BE as long as object A and object B ARE DESCRIBED AS HAVING DIFFERENT CHARACTERISTICS. It's that simple. Everything else is cheating.
“Finally, from so little sleeping and so much reading, his brain dried up and he went completely out of his mind.”
― Miguel de Cervantes Saavedra, Don Quixote