There are a few points to be made in response to the “Tasmanian problem.”
The first, as made by Josh, is that in principle, at least, Scripture’s use of “all” is seldom absolute, but nearly always conditioned by context. It would still be legitimate language to say “all men have sinned in Adam” whilst admitting the possibility that there was some isolated population nobody would aware of for two millennia that were not descended from him.
That would, of course, pose problems about the spiritual status of such a tribe once it was discovered: except that once the Tasmanians were discovered, interbreeding could and did happen, so the question would be virtually as academic as that of those outside the garden at the time of Adam: it would be Europeans who brought both the sinful heritage of Adam and the means of redemption from it, only in a way peripheral to the general tenor of Scripture.
Secondly, although it’s convenient to think of “universal Adamic mankind” existing by the time of Jesus (“1AD”), nothing much is changed if there were still non-Adamites then. The gospel was geographically restricted for centuries, and during all that time Adam’s genealogical influence would be moving ahead of it. The NT writers are speaking practically, and assuming the providential control of God over human affairs past, present and future.
Thirdly, there is a vast difference between scientific findings that appear to disprove a biblical truth, and those that appear unable to prove it. Science cannot prove the existence of the biblical Patriarchs - or of Adam himself, for that matter. But theology has no need of scientific proof for historical matters, and little in history can be so proved. What possible weight can the argument “Science cannot demonstrate this contingent event, so it is not true” have?
The problem with YEC Adam is that not only genetics, but the whole range of historical sciences, appears to rule it out except by ad hoc fixes like the false appearance of age. In other words, science appeared to dis prove it. But the fact that science considers a Tasmanian crossing unlikely is not evidence, still less proof of the negative.
As a parallel case, it is extremely unlikely, in absolute terms, that the first monkeys rafted to South America across a wide ocean, but that rafting is the standard explanation for their presence there now. If, by chance, that first raft-load of monkeys had died out after arrival, what would be the value of a scientific claim that the journey was “unlikely”? We knew that already, but it happened nevertheless in the real world. What makes a human voyage from Australia, or India, or China any more or less incredible than the origin of the new world monkeys?