What principle differentiates the bystander’s act from the surgeon’s act? Are both the fetus and the innocent person merely in the wrong place at the wrong time? I do not think so.
Prior to the act of the bystander, the innocent person’s life in not in imminent danger. The bystander deliberately involves the innocent person in something lethal to his life in order to further his purpose. The bystander is the direct agent in bringing about both the imminent danger of and, indeed, the death of the innocent person.
Prior to the act of the surgeon, the innocent fetus’ life is already in danger. So also is the mother’s life. To save the mother’s life, the surgeon excises the mother’s diseased tissue. The surgeon’s act does not introduce a new risk to the vitality of the fetus.
Unlike the act of the bystander, the surgeon’s act does not introduce or materially change the risk to the life of the fetus. (Although one may argue, depending on when the surgeon acts, that the surgeon’s act accelerates an inevitable death.)
Again, as I do not see any contradiction in VS 57 and CCC 2258, I do not see a need to reconcile.
The sequence I offered aims to flesh out the moral object of the act as described in the OP. The moral object does not involve the second font, intent. The moral object does consider foreseeable outcomes but does not rank them in importance. The importance of a particular foreseeable outcome is the consideration in the matter of intent. One may not intend an unforeseen outcome. Likewise, a deliberate (voluntary) act may not ignore or dismiss the moral responsibility for any foreseeable outcomes.
As such Step 5 in the sequence (If the bystander throws the switch then he immorally claims the right to destroy an innocent human being) remains valid as “unwanted” or “desired” do not apply in determining the moral object of an act.