We can't do it [create life in a lab], in a million years
Why do you say that? We'll probably be doing it in 50-100 years.
we assume it can happen in billions of years.
We assume that abiogenesis might have occurred spontaneously in earth's early history, not that it did. That's reasonable. On what basis could anybody say that it couldn't have happened? Because they can't imagine how it could have happened?
we assume it's not impossible
That's the default position for all that has not been found to be impossible. Whatever is not known to be impossible is possible by default, but in the loosest sense, that is, which includes things that may in fact be shown to be impossible someday, but not yet.
for one to say it is not possible, they have "an axe to grind"
To call impossible that which has not been shown to be impossible is to believe by faith. That's the person with the agenda.
We can't test the supernatural, but one who says there is evidence, and the supernatural is both probable, and possible, is irrational
We can call the supernatural possible, although you should be clear about what you are referring to. There's a good argument that anything that exists is just another aspect of nature and therefore natural, even if it's a god existing other alternate laws of nature as yet unfamiliar to us.
one who dogmatically dismisses the supernatural as imaginary, has no axe to grind - they are rational
It is ration to dismiss that for which no evidence is offered.
"You don't understand head nor tail how your theory works, nor how it's supposed to work. You can't even explain it without sweeping all the problems under the rug. After you do that, it's even more of a problem explaining it."
Only creationists express such opinions. Evolutionary science is here to stay. Like the heliocentric theory for the solar system, there is no realistic hope of it ever being overturned.
Consider the implications of a falsifying finding being uncovered, such as a partially digested human being in a dinosaur's belly. The old data that strongly suggested that life on earth - millions of pieces of evidence from multiple fields of science such as comparative anatomy and biochemistry, biogeography, genetics, and paleontology - that old data doesn't disappear.
It would need to be reinterpreted in the light of the new finding, and no other interpretation occurs to me than that an extremely powerful and deceptive agency intended for us to be deceived into believing that life had evolved on earth to the extent that it buried strata of life forms that never lived such that the most primitive appearing would be found deepest and with a combination of radionuclides that made them appear oldest, with progressively more modern forms appearing in shallower strata.
The theory is healthy and in no danger of being overturned.
And yes we know how and why evolution occurs. The mechanism is well understood.
Your task is hopeless. You are trying to persuade rational skeptics well-trained in critical thinking to abandon a system of ideas that unifies mountains of data from a multitude of sources, accurately makes predictions about what can and cannot be found in nature, provides a rational mechanism for evolution consistent with the known actions of nature, accounts for both the commonality of all life as well as biodiversity, and has had practical applications that have improved the human condition in areas like medicine and agriculture - and replace it with a sterile idea that can do none of that, one that can't be used for anything of value?
The evidence is interpreted in a way that gives support to the presumption of common descent.
The evidence can be interpreted in no other way apart from the deceptive agent I just alluded to. Either life evolved on earth from a last universal common ancestral population of primitive cells, or somebody went to a lot of bother to make us think so. Both scenarios exclude the possibility of a loving god that wants to be known, understood, believed, and worshiped by man being the source of the tree of life we find today.