I think where your argument breaks down is in the fact that scientific theories are tentative. You Theory of Roof Wetness does not have the same muster as a complete logical statement but it IS PARTIALLY USEFUL for describing the natural world. If my roof is wet, it likely that it has rained because the other means for my roof to get wet are much less likely than rain.
You mistake is in assuming that scientific theories must be:
1. Complete
2. As stong as a logical statement
Science will settle for, "the best aproximation to reality that we can get at the moment". Logic will not settle for such a thing.
The other problem I have with you OP is that it is often easy enough to provide a counterexample to the fallacy of affirming the consequence. Therefore if there were some science based upon affirming the consequence in a bad enough way then it should be seemingly easy to refute the conclusion.
For example, I could simply wet my roof with a hose as a counter example to my roof is wet => rain.
To invalidate the argument in the case of the marsupial isolation, all one would have to do is prove that marsupial fossils have a wider distribution around the world. Without that evidence, there is no reason to doubt the conclusion.
Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)
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