js1138 at 1570 common descent is just the current best interpretation of evidence. It is unrelated to biogenesis or to the dynamics of evolution.
All evidence at hand is consistent with the descent of all multicellular organisms from a common population. But evolution would still happen if this were not the case.
What you said:
That is the point. One cannot say it is a continuum - a “tree” - in the face of the possibility of multiple episodes of abiogenesis or biogenesis.
Here's how your logic seems to work: "John died with two bullets in his head and no other ecivence of illness or injury; there are many ways that people can die; therefore, because there are many ways to die, we cannot say John died from bullet wounds."
Evolution is a process. The process works regardless of the specific history on a specific planet.
Our best available knowledge of common descent goes back not much further than 500 million years. When you get to the era of single celled organisms, the word "descent" has no clear meaning. Single celled organisms share and acquire DNA in ways that are not really analogous to bisexual reproduction.
The known history of life goes back 3.5 billion years. In other words, the vast majority of the history of life is blurred or erased.
Your generalizations are not helpful to understanding. You have attempted to sidetrack the discussion from its starting point, which was the claim that a quote was fabricated. Are you willing to concede that Darwin never said life can only come from life?