Every US citizen has a Constitutional right to sue for libel (common law right existent at the time of ratification).

That’s— no. That’s not how either of those things work.

First, something being a common law right doesn’t make it a constitutional right because constitutional rights are, y’know, part of the Constitution. There is nothing in the Constitution or in case law around the Constitution that establishes a constitutional right to sue for libel beyond the fact that you have the right to file a lawsuit over just about anything (though not without consequence, and there’s no guarantee the lawsuit will get past the pleadings stage).

Second, there is no federal common law. Historical British common law is handled on the state level, not the federal level. And even where there was a common law right existent at the time the Constitution was ratified, that doesn’t mean it is still recognized in any given state, and those rights can differ between states. For example, at the time the Constitution was ratified, there was still an established common law right to trial by combat, but no state still recognizes that right.

Now, there are written state laws in all 50 states (plus DC) that allow for someone to sue one who defames them (which includes both libel and slander), and there is case law that allows them to coexist with the 1st Amendment protections of freedom of speech and of the press, but that is not a constitutional right to sue for libel so much as a limitation of certain constitutional rights to allow one to sue for libel. Also, there is nothing there that establishes intermediary liability for libel.

Such a right cannot be negatived without a Constitutional amendment.

For actual constitutional rights? True, with the caveat that interpretations of those rights are done by the judiciary. For historical common law rights? Absolutely not. If there is a conflict between a statute or law (by the state or the federal government) and common law, the statute or law almost always overrules the common law. For the right to sue for libel? Hell no. That is neither a constitutional right nor a common law right; it is established by written state statutes and curbed by the 1A, the corresponding state constitution, and by federal statutes as interpreted by the judiciary (thanks to the Supremacy Clause).

If active curating (a form of the social medium platform’s own speech) occasions an act of libel, […]

How can deciding whether or not to host someone else’s speech be libelous? While the decision not to host something is speech by the platform that is protected by the 1A, I fail to see how it could possibly be defamatory speech.

[…] 47 U.S. Code § 230 is unconstitutional if it immunizes a social medium from the consequences of its own libelous speech.

There is no right (constitutional or otherwise) to impose legal consequences on someone because they failed to remove someone else’s libelous content or because they did remove someone else’s non-libelous content. There is a statutory right to sue anyone (including the makers of a social media platform) for their own libelous speech, and that isn’t immunized by §230, but curation of user-generated content cannot itself be libelous. Only statements of fact can be libelous, and actions—even when they are expressive, like with curation—are not statements of fact.

Once SCOTUS declares § 230 unconstitutional (at least four Justices seem poised to do so), my plaintiff class will on remand prevail and bankrupt the social medium platform.

First off, you need 5 to prevail in the Supreme Court, so even by your own account, this is far from a foregone conclusion. You have also failed to state a case that §230 might be unconstitutional.

If § 230 does not immunize a social medium platform from the consequences of its own libelous speech in the form of active curating, my plaintiff class will prevail against a social medium platform and bankrupt the social medium platform without the detour to SCOTUS.

Again, active curation cannot be a statement of fact, so it cannot be libelous speech. As such, you have failed to state a claim against the platform at all.