There is room to say that this itself is the answer: because he violated the word of the Merciful One. But how did he violate it if the prohibition never took effect? Perforce, it must have taken effect and then lapsed, and that is why he violated the word of the Merciful One.

First of all, in the question you agree that they understood it that way. Now we have to discuss whether the answer changes that. Apparently not, because if it takes effect then he receives lashes for the legal effect that he brought about, not merely for violating the word of the Merciful One. That is a full-fledged prohibition, so why should it matter that it lapses a moment later?

In particular, one must say this if one accepts the well-known principle of Rabbi Shimon Shkop: any legal effect such that, if it takes effect, then it does not take effect — does not take effect.

Exactly — I also thought so based on Rabbi Shimon Shkop, but I wanted a proof even according to those who disagree with him.
Many thanks for your answer.
As for your actual point: if the prohibition and the word of the Merciful One are not to bring about the forbidden legal effect, then if it is not effective and never takes effect at all, he did not violate the word of the Merciful One. So I am inclined to say that it did take effect and then lapsed, and that is why he violated the word of the Merciful One. Not as people simply learn that the word of the Merciful One means the act itself without the prohibited legal effect, for the Torah did not prohibit the act as such, but rather the legal effect.
Forgive me for digging into this again and again.

What about the possibility that it takes effect for a moment and is nullified, and then during twilight this coming Friday it takes effect again for a moment and is nullified again?

A simple proof, in my opinion, is from Tosafot in yesterday’s Daf Yomi page (Gittin 14). The Talmud explains there that one who takes hold of pe'ah for a poor person enters the dispute between Rabbi Eliezer and the Sages, and the Talmud wants to explain that they disagree about one who seizes property for a creditor in a case where this harms others. The Talmud rejects this, because it may be that the Sages hold that such seizure is effective, and they only derive from a verse that it is forbidden to acquire pe'ah on behalf of a rich person.
And Tosafot there ask: according to the view that whenever the Merciful One said not to do something, if one did it, it is effective — then the rich person has in fact acquired it after the fact.
Now, if we say that even according to the view that it is not effective, the act takes effect and only afterward lapses because of the transgression, then presumably here the rich person’s ownership should not lapse because of the violation of the one acting on his behalf. His act of transfer has already been completed, and now the ownership stands on its own. Tosafot should then have asked their question even according to the other view.

I did not merit to understand your reasoning. Do you really think the Torah lacks the power to undo the rich person’s acquisition that came to him through the agent’s transgression? I am astonished. All the more so whether it would be retroactive or only from that point onward.

“Violated the word of the Merciful One” does not mean that he actually brought the prohibition into existence, but that he performed an act whose purpose was to create a prohibition. Therefore, even when the prohibition concerns the result, if a person performed the act and the result was not produced, he still violated the word of the Merciful One. On the contrary, if it was produced and then nullified, then he violated the prohibition itself, not merely the word of the Merciful One.
So there is no reason to distinguish here between a prohibition of action and a prohibition of result. In fact, we are dealing only with prohibitions of result, because an action cannot be undone, so this discussion is not relevant to it.

A.] Thank you very much — that is very acceptable to me.
That is, the word of the Merciful One is not the prohibition itself, nor its essence and definition, but only the language of the Torah’s command and warning about what not to do in actual reality, even though the purpose of the prohibition is something else. It is still somewhat unclear to me from where we learn the definition, essence, and purpose of the prohibition.
B.] Still, there is no absolute proof here, because one could also say otherwise, as above: that the very word of the Merciful One, meaning the command in the Torah, is the body of the prohibition itself, and included within it is the purpose of the prohibition — namely, not to bring about the end result of this or that act. If so, it would necessarily follow that the lashes are for the end result, not for failing to heed the warning and refraining from the act that leads to that end result, because everything is included in the command written in the Torah. And once again we would have to say that the prohibition took effect and then lapsed immediately.
C.] And after some thought, perhaps it seems that this is indeed forced from the fact that Abaye disagrees and holds that it is effective, because otherwise why are there lashes? And why is he not satisfied to say that the lashes are for the prohibition that took effect for a moment? Why should we care that it lapsed immediately afterward? It is strained to say that he holds that lashes apply only to a prohibited legal effect that continues after taking effect, but not one that lapses immediately. Perhaps it depends on the definition and essence of the prohibition, and here we are dealing with prohibitions that are fit to continue after taking effect, and therefore only that is what the prohibition and the lashes refer to.
D.] But still, if not as I have suggested above, but rather in the simple sense as stated earlier, it remains unclear why Abaye is not satisfied to say that the lashes are for violating the word of the Merciful One, which is the very command itself. Why does he need the prohibited legal effect also to take place?
In short, according to this it comes out that the whole dispute is whether the lashes are for the transgression as disobedience to the Torah’s command in reality through the act itself, or whether the lashes are for transgressing the very forbidden object-state and essence of the prohibition.
E.] What seems to me after all this is to add one more new point.
What bothers Abaye — is it the very fact that he receives lashes, meaning: how can he receive lashes if it is not effective and the legal effect never takes place, and therefore Abaye is forced to say that it does take effect and is effective? And to that, Abaye answers that there is still a reason for lashes even if it did not take effect.
But then it is difficult for me: why do they also disagree there in the case of separating poor-quality produce as tithe for good-quality produce? That is only a positive commandment, and there are no lashes. So why should it bother Abaye here? Let him say, as Rava does, that it is not effective and does not take effect, since in any event there are no lashes.
This forces me to say that what bothers him is the very command and warning: what purpose does it serve, and why was it written, if nothing takes effect through the forbidden act? Where there are lashes, the lashes show that he violated the prohibition; and where there are no lashes, as with a positive commandment, then the positive commandment itself bothers him: why was it stated, if in practice, even when one acts contrary to what is required by that positive commandment, still nothing takes effect? After all, the whole prohibition is the end result and the legal effect — and if it will not take effect, what is the point of prohibiting and commanding? And where there are lashes, the emphasis is on the lashes, because through them it is evident that he violated the prohibition and that the prohibited legal effect took hold; and if it did not take hold, there are no lashes, so what is the point of the warning? On this Rava disagrees: even if the prohibition does not take effect, the warning and command are still justified even in the case of a positive commandment, because even merely doing the act that runs contrary to the command is itself enough reason for the positive commandment to have been written, so that this should not be done even if the intended result does not take effect. Still, it remains unclear whether this is not really two different disputes expressed with the same words. This requires further analysis.