It's not often that I find logic at odds with Walter Block, but here he goes out of his way to ignore that a pregnant woman has acted to bring about her pregnancy, stripping her of moral agency and reducing her responsibility for her own actions to those of, ironically, a baby.

The woman who refuses to carry her fetus to term is in exactly the same position as a person who refuses to rescue a drowning swimmer.

Only if the person refusing to help was the one who pushed the drowning swimmer into the water.

This operation was conducted without the permission or even knowledge of either “patient.”

This either presumes that women don't know that sex causes pregnancy or that taking an action does not imbue responsibility for the consequences of that action.

This case is not analogous to the one where an individual is invited on an airplane trip, and then halfway, while he is up in the air, the owner states that the invitation was only for fifteen minutes and that the time is now up … so stop trespassing and leave forthwith, sans parachute. There is an implicit contract in force in that instance.

It is, actually, in that sex causes pregnancy and consent to sex was given, but let's assume with Mr. Block that women are imbeciles and alter the analogy to match:

A certain pilot takes off on a trip from LA to NYC. Above Utah, the pilot discovers a stowaway on her plane, a child who tells the pilot that they were placed on the plane by someone else. What can the pilot do with the child? Certainly we would consider throwing it out of the plane to be murder and no court or jury would be interested in a defense of "I didn't sign a contract with the child to provide them with safe travel".

the fetus aggressor, albeit not purposefully, is the initiator of violence.

Ah, so now having removed moral agency from the pregnant woman we place it on the shoulders of the unborn.

The anti-abortionist position, to be logically consistent, would have to hold this action as premeditated, first-degree murder.

Your terms are acceptable.

the woman’s right to her property – that is, her womb – must be held above the valuable life of the fetus.

Must it? This is begging the question. It assumes that we value the right to property above the right to life without justifying the assumption.

a true compro­mise between the pro-choice and pro-life camps. The former gains half a victory: the woman may rid herself of the fetus, as is desired by the pro-choicers, but they will be disappointed in that she does not also have the right to kill it.

If the woman does not have a right to kill the fetus then evicting it leading to its death could be considered negligent homicide. This is far from the compromise Block thinks he's crafted.

This compromise, or eviction position, is a true intermediary between the two more well-known abortion theories, pro-life and pro-­choice.

And this is the reason why. Block is grasping here, trying to position himself, and libertarians as a whole, as the "reasonable third choice". Given that this article was penned about 20 years ago we can see that being the "reasonable third choice" has gotten libertarians nowhere. Conversely, the democrats painting Trump (a 90's New York democrat who found himself in alignment with republicans some 20 years later) as extreme has not scared away his supporters - if anything it has only hardened their resolve to follow the loudmouth do-nothing into likely disaster.

Some things are worth planting a flag on and nobody cares about the compromise party. If libertarians consider, as Block does, abortion to be immoral because it causes the death of the baby then grow a pair and plant your flag on that moral stance.