Question

Good evening,
In the Torah portion of Nitzavim:

“The hidden things belong to the Lord our God, but the revealed things are for us and for our children forever, to carry out all the words of this Torah.”
Rashi explains:
“The hidden things belong to the Lord our God — and if you say: what is in our power to do? You punish the many for the sinful thoughts of an individual, as it says (verse 17), ‘lest there be among you a man…’ and afterward (verse 21), ‘and they will see the plagues of that land.’ But no person knows the hidden thoughts of his fellow. I do not punish you for hidden things, for they belong to the Lord our God, and He will exact payment from that individual. But the revealed things are for us and for our children — to remove the evil from our midst. And if we do not execute judgment against them, the many will be punished. There are dots over the words ‘for us and for our children,’ to teach that even for revealed sins He did not punish the many until they crossed the Jordan, after they accepted the oath at Mount Gerizim and Mount Ebal and became responsible for one another.”
The expression appears in tractate Shevuot 39a: “And they shall stumble, each man because of his brother” — a man because of his brother’s sin; this teaches that all Israel are responsible for one another. And the Talmud establishes that this refers to a case where they should have protested and did not. The responsibility spoken of here is that if Reuven sinned and Shimon was in a position to prevent him or protest against him, and did not prevent him, Shimon is punished.
 
In tractate Rosh Hashanah 29 we learned:

“Ahavah the son of Rabbi Zeira taught: In the case of all blessings, even though one has already fulfilled his obligation, he may still discharge another’s obligation, except for the blessing over bread and the blessing over wine; if he has not yet fulfilled his obligation, he may discharge another’s obligation, but if he has already fulfilled it, he may not discharge another’s obligation.”
And Rashi explained: “For all Israel are responsible for one another regarding the commandments.” And the Ritva elaborated:
“Ahavah the son of Rabbi Zeira taught, etc. Meaning: with respect to all blessings over commandments, even though one has already fulfilled his obligation, he may still discharge another’s obligation. For although the commandments are incumbent upon each individual, all Israel are responsible for one another, and they are all like one body and like a guarantor who pays his fellow’s debt. This includes as well the blessings of the Shema, where the prayer leader discharges even one who is proficient, provided it is in a congregation, as is proven in the chapter ‘One Whose Dead Lies Before Him.'”
Seemingly, a very interesting leap has been made here. From the verses about punishments, one can learn at most an obligation to prevent and rebuke, and one who did not prevent is punished, but clearly he is not considered an actual sinner, since the Talmud in Shevuot says that he is liable to karet while his family is not liable to karet. Here a further leap was made. Responsibility becomes an actual obligation upon each person regarding his fellow’s commandments, so that if my fellow did not perform a commandment, I can perform it on his behalf and discharge his obligation. Seemingly, this is a major novelty, and it is not entirely clear how it follows from mutual punishment.
 
With blessings,