Esther also was not married according to the understanding of the medieval authorities (Rishonim) — they do not accept the exposition in tractate Megillah that she was married — so there was no forbidden sexual relation. The whole difficulty in tractate Sanhedrin regarding Esther’s act stems from the fact that it was done in public, and the answer is that it was for their own pleasure. That answer does not work for the three cardinal sins (Shulchan Arukh, Yoreh De'ah 156).

It should say 157. In the plain sense of the Rema there, it is speaking even about a married woman. True, the medieval authorities (Rishonim) and the commentators there wrote that perhaps it follows the view that she was unmarried. But that is not the plain sense of the Talmudic text or of the Rema. And according to the views that she was unmarried, perhaps indeed the words of the Noda B'Yehuda do not fit. One should also discuss whether the case of the Noda B'Yehuda would be considered public.

In the past I dealt with a question involving a Jewish intelligence agent engaged in saving the State and Jews from various dangers,
and at one point they ran into a moral question.
It concerned a precious Jewish woman who could have brought information worth gold for saving many Jews by linking up with one of the heads of the terrorists who was traveling abroad.
They debated whether to order her to offer herself for that purpose,
since only that way, apparently, could she gain trust and get her hands on the information.
The dilemma was ethical and moral, and also whether it was lawful to give such an order for so lofty a goal.
Meanwhile, she acted on her own and spared the commanders the question, and thank God the information came through and many were saved.

There she was single and it was done privately.
Is it reasonable that if Jewish law permits such a thing in that situation, that is also a moral-ethical and legal-command answer? Or not?

There is also room for doubt regarding the definition of passive ground:
1.
Does it mean that indeed she should not act at all, not even while he has not yet begun the act itself but is still hesitating,
rather she should leave him be, and if he advances to the act, let him advance — but she is forbidden to act by enticing, touching, speaking, and the like?
2.
Or is she permitted to increase words and surrounding actions to encourage him toward the act, just not the actual act of penetration?
3.
Or is even penetration permitted for her, since in principle this is a kind of act where what is required is only his act, and in this category of transgression, in principle and in practice, her act is not “required”?

Sorry for the length,
but these are important questions.

I didn’t understand how this differs from what I wrote.
The definition of passive ground applies only to what happens at the time of the act itself, not to the preliminaries. The preliminaries are, at most, a matter of “do not place a stumbling block.”

Much appreciated.

If Jewish law permits, and perhaps even recommends, the above in such a case,
is it reasonable that this is also the ethical-moral and legal-command standard, absent an explicit contrary instruction?

The moral question depends on assumptions. There is a halakhic assumption that such an act is very severe. Morality does not deal with the severity of the transgression, but with the price the woman pays (psychologically). So the two are not really comparable.