The theory becomes so flexible and so accommodating as to become meaningless
Any theory of life, evolution or creationism being no exceptions, has to account for the
full diversity of life on Earth. It has to be flexible enough to accommodate an explanation for
every single living thing. You act like it's a fault that the theory has such wide-ranging explanatory power, that if the theory were
true there would be living things it couldn't explain, but that makes no sense.
And it's a double standard, as well; if we observed your hypothetical instant-second-head, and asked
you to explain it from the perspective of the "theory"
you advocate, what would you do?
Throw up your hands in defeat? I doubt it. Wouldn't you suggest some variant of "God did it"? I mean, couldn't you say that for literally anything at all? "God made it that way." Can you explain why that doesn't make
your "theory" "so flexible and so accommodating as to be meaningless"?
By the curious standard you're setting up, the most meaningful theory would have to be the one so inflexible as to explain absolutely nothing at all. Why not?