And one could further say that the prohibition is being present in a gathering, not the act of gathering itself (as opposed to the case of “five on a bench,” where the breaking follows immediately upon the sitting), and therefore each member of the group should have separated himself from that evil congregation. By rights they should all be fined, for not drawing back their feet.
As for the idea of a “law of doubt” in Rabbi Shimon Shkop, I never understood: according to him, from where did the Talmud derive this astonishing novelty that the kiddushin takes effect? We learned that one can betroth a particular woman, and plainly if he did not perform the betrothal according to the full protocol, he did nothing at all. It is like someone curling his hair and expecting kiddushin to take effect.
In Sha’arei Yosher (3:22) he writes about this: “The cause of the legal effectiveness of kiddushin is the act of kiddushin—the giving of the money and the declaration,” and it seems he means to exclude seclusion and final intention toward a particular woman. But it is still difficult: from where did the Talmud get this, and the whole mysterious idea of legal effect hanging in the air without finding any place to rest its foot? (And in Sha’arei Yosher there he elaborates on the matter of retroactive clarification.)
Similarly, two firstborns whose heads emerged at once. Neither one is the offspring that emerged first, so both ought to be ordinary, unconsecrated animals. Though here one can inquire at first glance whether firstborn status merely consecrates the firstborn and has nothing at all to do with the other offspring, or whether it also exempts the mother’s womb with respect to offspring yet to be born. If firstborn status merely consecrates the firstborn, then if their heads emerged at once, neither is a firstborn and so both are ordinary animals. But if firstborn status (and the birth of a female) also exempts the womb, like terumah with untithed produce (where produce grown from untithed produce is forbidden), then if their heads emerged at once, neither is a firstborn, and therefore neither of them is ordinary.

Shulchan Arukh, Choshen Mishpat 381:

“Five people sat on a bench and it did not break. Then another came and sat on it and leaned on them and did not allow them to get up, and it broke. Even though it was fit to break under them before he sat, since he hastened its breaking, the last one is liable, for we say to him: had you not leaned on us, we would have stood up before it broke.”

Meaning, if he had not leaned on them, they would all be liable, because they should have gotten up. However, the Rema there implies a bit that this is talking about a case where the last one to sit did not know he was endangering the bench, because if he was an especially heavy person (and should have known that), then they do hold him liable. And in the case of the fine, the last ones who entered should have known that they were breaking the law and endangering everyone, both with the fine and with contagion.

A great deal of pilpul can be made here, and distinctions can be drawn from the case of “five on a bench,” but this is not the place.