It is somewhat of a strawman in that you are implying that homologies are the only presented evidence of common descent, and also that all scientific claims must be able to be expressed in valid formal logical form. Really, very, very few things can actually be "proven" in such a rigorous manner as you seem to be requesting, rather scientists look at the available data and try to figure out which explanation better accounts for this data. Very rarely do such explanations stand up to being expressed in the form of symbolic logic, and that is why scientific ideas are open to change, but that doesn't mean science is a load of bull.
If your question is specifically about the argument form you presented (p—>q, q, ∴p) that is invalid because it is the converse error. However, if you are attempting to cast doubt on ideas of common descent with this post, you are yourself committing a logical fallacy. Argumentum ad logicam.
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