Geirsson and Losonsky, in Plantinga and the Problem of Evil (from your "P.S." statement) fail at least two ways in attempting to address Plantinga's argument:
- Counter-Factuals (CFs) are possible actions. But all possible actions are not guaranteed to be actualizable actions.
- They miss a piece of Plantinga's argument which they even quote: ".. he will take at least one wrong action"
For those who don't wish to read all of Plantinga's argument and the response from Geirsson and Losonsky, the particular passage I refer to here is regarding an example from Plantinga in which a person with free will, Curley, is presented with an opportunity to accept a bribe, and he accepts. This is considered a moral failing (evil in the world) by Plantinga, but you could substitute any other moral failing or free will action that you believe brings evil into the world.
Point 1: Counter-Factual Actualization
Regarding possible worlds and CF actualization: both your question and Geirsson and Losonsky are assuming that all CFs are actualizable. In other words, there is an assumption in the argument against Plantinga that there is at least one possible world where all CF possibilities could actually occur.
Consider Plantinga's example of Curley and whether or not he will accept the bribe: Plantinga is stating that a weakly actualized world in which Curley is presented with the same opportunity to accept the bribe will result in Curley always accepting the bribe. That is the free will choice that he makes, as a morally free agent. He calculates the moral situation and chooses wrong. Presented with the same circumstances and the same ability to freely choose between good and evil in this particular situation, he will always choose evil and accept the bribe. Free will doesn't mean we can willy-nilly choose a random action from among those that are available. It simply makes us the agent that does the choosing between the right and the wrong way without compulsion to choose one or the other by circumstance or God forcing our hand. It doesn't mean that given the same circumstances in another possible world that we would choose differently! If we did, that would make us a different person - a different moral actor.
While there is a possible CF action where Curley does not accept the bribe, Plantinga's argument relies on the fact that Curley, given the same circumstance, will always accept the bribe - his will/nature does not and won't change unless his free will is destroyed through strong actualization.
Edit: Plantinga's word for this is "Essence". Curley's Essence leads him to make the free will choice of always accepting the bribe in that circumstance. "Essence" is distinguished from "Person" as explained here: Alvin Plantinga's Free Will Defense
Therefore, while there is a possible CF action, there is not a realizable CF action. The only way of obtaining a possible world where Curley does not accept the bribe, according to Plantinga's argument, is for God to strongly actualize a world with the circumstance's changed in a way which removes Curley's freedom of action to the point that God has, in reality, removed Curley's free moral choice, his free will. God must either not allow him to choose by removing the circumstance so there is no choice, or God must directly force him to choose differently.
The long and the short of it is, that according to Plantinga's argument, there is no possible world where Curley both has free moral agency, and does not take the bribe, due to Curley's nature. Now, there are plenty of people other then Curley, who put in the same circumstance, will not take the bribe. So there are possible worlds where we can find a person with free will who will not take the bribe - I will address this in the second point, but for the first point, we are only dealing with Curley, and his free will choice, given Plantinga's example, is that with the circumstances placed before him, his free will choice is to take the bribe. No CFs exist that can be actualized in another possible world for Curley, in particular, where he does not take the bribe, without God changing the circumstances in a way that does violence to his free will choice.
In order to find a possible world where Curley does not take the bribe, you must remove his free moral agency - you must change his nature, his will, so that he no longer desires the same outcome from the situation. Such a possible world does not exist while keeping his free will intact.
The only other option is for God to not create Curley. For than, see point 2.
Point 2: ".. he will take at least one wrong action"
The second point is that Plantinga doesn't require for a free will agent to take the bribe in the given scenario in order for them to be Transworld Depraved (TD). While we know that Curley is TD because he does (and always will) accept that bribe given the same circumstance, but that doesn't mean that all moral agents must also accept this same bribe to be TD. After all, if every moral agent only accepted the bribe, then it would be easy to argue that free will didn't truly exist.
But Plantinga doesn't require that every moral agent accept the bribe, given the same circumstances. Plantinga only requires that any given moral agent must take at least one wrong action to be TD (quoted from the end of his example).
God could choose to actualize a world without Curley, because God knows that Curley will bring evil into the world by accepting the bribe. He can certainly find a possible world with free moral agents other than Curley who will not accept the bribe. The key, though is that if these other actors fail morally in any other choice, then they also will be TD. To pull this off and create a world with both free will and no evil, he needs to find a world with moral actors that won't choose any wrong action. Plantinga's argument only requires that any given free will moral actor fail on one possible moral choice for them to be TD.
So, let's take Mary, who chooses the good and does not accept the bribe. That only shows that it is possible that she is not TD. It does not prove that she is not TD. We must put every other possible situation in front of her, and if she fails morally at one of them, then she has taken (or would take) at least one wrong action, and is TD. Plantinga's argument only requires the possibility that all moral actors are TD. To defeat his argument, it would have to be proven that this is not possible - that there must be a possible world where there are free moral agents will not undertake a single wrong moral action.
I actually believe that Plantinga's defense would stand up even if it can just be shown that a single moral actor would always choose wrong; but it is a stronger argument if it remains a possibility that all moral actors must be TD.
Bonus Point: Time Segmentation? What the...
The remaining thrust of Geirsson and Losonsky's argument against Plantinga's defense relates to actualizing segment's of time. While other's appear to have accepted this at its face and addressed this directly (see Transworld Depravity and Unobtainable Worlds by Richard Otte), I believe time segmentation and trying to build a perfect good world is quite irrelevant. Can we build a world without evil by creating Adam and then ending the world before he sins? Sure, but that destroys the point completely. If the point is to defend the God of the Bible (or an all-powerful, all-knowing, always good God who creates man to have a relationship with God, if you prefer), then attempting this time segmentation by cutting the timeline short does violence to God's purpose with respect to creation - to create free moral beings that are truly capable of love and relationship with him. If that is the ultimate good that required free will moral actors to be created, then it should also be a part of the equation.
I understand that in the end the time-segmentation argument is an attempt to string together all the time segments in which only good occurs into a possible world where there is no evil done - but again, that cannot be achieved because it relies on the assumption that all CFs are actualizable for a given moral agent with free will. This argument explodes because it forces a person to be different then they choose to be in order to have a possible world where they choose differently. It is just a disguised form of strong actualization - trying to do piecemeal what Plantinga's base argument disallows in its definition of strong actualization.
