Does the Rabbi really think there is no problem at all with speaking during the reader’s repetition? Are we not obligated because of the authority of the Sages, and even if the reason has fallen away, the enactment has not fallen away?

The enactment is to have the reader’s repetition, but there is nothing here through which to fulfill an obligation. So why should speaking be forbidden? At most there is an issue of showing respect for the prayer and the occasion, but not a prohibition.

Does the person speaking need to make sure that there are at least nine people besides him listening to the reader’s repetition, or is that not indispensable?

Did the Rabbi really overlook what is written in the Shulchan Arukh, Orach Chayim 124:7: “One who talks sins, and his sin is too great to bear”??
Still, your claim that since there is no obligation here to fulfill, it is permitted to speak, is quite strong. And in truth, because of that I’m now wondering why it is actually forbidden to speak during the reader’s repetition.

I am not aware of any need to make sure everyone is listening. There are ten present, and therefore there is a minyan.
I didn’t overlook it at all. “His sin is great” is exactly what means that there is no halakhic prohibition here. The Shulchan Arukh is not really strict about distinguishing between proper conduct and a halakhic prohibition.

The Shulchan Arukh (124:4) rules that the whole congregation must be silent and concentrate on his blessings and answer amen, and if they do not, it is close to saying that the blessings are in vain, and each person should see himself as though there are not nine besides him and should concentrate on the blessings. Don’t we see from this that according to the Shulchan Arukh, the meaning of a minyan is specifically that people are attentive and answer amen to the blessings, and that their mere presence is not enough?
In the end, if the one who speaks has “a sin too great to bear,” it is hard to define the avoidance of speaking as merely proper conduct, which makes it sound as though it is preferable not to speak, but it’s also fine not to be careful about it.

See Magen Avraham, subsection 8, in the name of Rabbi Menachem Azariah of Fano, that it is enough to answer amen at the end of the blessing. And so too it is proven from סעיף 3 in the Shulchan Arukh regarding those who prolong their prayer, that there is no need to wait for them, and likewise for an important person. And there they also permitted saying supplications and studying during the reader’s repetition. If there were a need to fulfill an obligation through it, there should be no difference between saying supplications or studying and ordinary conversation.
But in my opinion, simply speaking, there is no need at all to fulfill an obligation with these blessings. The reader’s repetition needs to be said in the presence of ten, but they are not supposed to fulfill an obligation with the blessings, because they have no such obligation—unless they do not know how to recite the silent prayer themselves, as written in subsection 1 there.

“‘His sin is great’ is exactly what means that there is no halakhic prohibition here.”

On the contrary—if his sin is great, that implies he did something forbidden… I’d be happy for an explanation, because I didn’t understand.

When there is a halakhic prohibition, I would expect the Shulchan Arukh to write that it is forbidden, full stop. That is enough to stop me from doing it. When it writes that “his sin is great,” it sounds like it is trying to motivate us to act this way even though there is no halakhic prohibition. Punishment and sin also exist outside Jewish law—for example in moral transgressions and in conduct beyond the letter of the law.

And now that we’ve come to this—that “his sin is too great to bear” comes to teach us that there is no halakhic prohibition in it—does Cain’s statement “My sin is too great to bear” also teach that murder carries no halakhic prohibition? 🙂

Best regards, irony upon irony

Absolutely. In Cain’s case there was not yet any halakhic prohibition. That is despite the fact that the comment is baseless to begin with.

What difference does it really make whether it is forbidden from a halakhic perspective or a moral one? At the end of the day, it’s forbidden, no??

Are you talking about speaking during the reader’s repetition? This is not about morality but about an appropriate attitude toward prayer. But in any case it makes a great difference, because if there is a moral consideration in favor of speaking, then there is room to say that it overrides the non-halakhic prohibition against speaking.