You seem to forget that ancient myths can becataloged by the method of Comparative Mythology. You also seems to ignore that the ancient Myths of Creation deals with real cosmological knowledge and astronomical observations and not mumble jumble hearsayings.
Oh, I don't forget anything. Just kidding. But in this case, I'm not ignoring the context of the myths in question, particularly of those in Mesopotamia, as the OP set out. There is cosmological knowledge and astronomical observations in the creation stories among the myths in Mesopotamia; however, the flood story doesn't fit into the creation myth. The flood story happens afterwards, after creation. Its a separate myth, and can't be combined with earlier mythology.
And one can compare various mythology, but that doesn't necessarily mean that they are 100% the same, or even comparable. There is a danger in comparative studies because often one wants to merge the two objects into one. You can't do that.
No you can´t say that "early Native American believed in Tiamat" but they certainly believed in the same observation of the Milky Way to which they gave their special name and symbolism which computes to the Mesopotamian Tiamat.
Did they though? Nope. There is some similarity, of course, since we are talking about something that is observable around the world. But the ideas surrounding the Milky Way, or Moon, or sun, etc are quite different. To try to smash it all together doesn't work as it means one has to ignore the actual details of either mythology.
Regarding the "Dragon Tiamat tail": How would you otherwise describe a god with a tail?
Monkey, birds, reptiles, etc. There are many things with a tail. Also, the tail itself would not constitute a dragon. I can't look at a monkey's tail and declare that it is a monkey.
Quote from -
https://en.wikipedia.org/wiki/Tiamat
“It is suggested that there are two parts to the Tiamat mythos, the first in which Tiamat is a creator goddess, through a "Sacred marriage" between salt and fresh water, peacefully creating the cosmos through successive generations.
In the second "Chaoskampf" Tiamat is considered the monstrous embodiment of primordial chaos. Some sources identify her with images of a sea serpent or dragon”.
Some sources. That's key. Not all sources. A sea serpent and dragon are also different. She has also been identified as a regular, human goddess, and often time, isn't identified in any form. None of that matters though as I was specifically talking about the tail, and only the tail. The tail itself does not depict a dragon, and the depiction in the particular myth swings neither way.
Just like in the Norse Mythology and it´s Midgaard Serpent, Tiamat represents the Milky Way contours, which encircle the entire Earth.
But Tiamat doesn't represent the Milky Way. Her tail does. There is a difference.
These myths are really talking of cosmological and astronomical facts and even facts of the creation of the Milky Way itself. And these are of course facts for all humans all over the world.
We are talking about a flood story, not creation. The flood stories happen long after the creation. So to jumble every myth together, as you are doing here, doesn't make sense, as they are referring to very different things.
Also, the cosmological and astronomical facts you are talking about don't exist in all of these mythological stories. The creation is often very different depending on which myth you look at. Some don't even talk about the Milky Way at all, and when they do, its creation is different. In order to get to your conclusion, one must ignore the differences, and only look at the similarities, while combining a random assortment of myths together, which doesn't work.