What is formally required, in legal terms, to abolish it?
Can a person decide this for himself? Can a community rabbi abolish it for his community? Agreement of all / a majority of the Torah scholars in the world? A Sanhedrin?

That is not a simple question. In principle, the rule is that something established by formal count requires another formal count to permit it. This was established by the Sanhedrin, so a Sanhedrin is needed to revoke it. Moreover, this is a rabbinic law, so it would require a Sanhedrin greater in wisdom and number than the one that enacted it.
But here there is a view among the medieval authorities that in the Land of Israel it is not required, meaning that according to that interpretation there was never any enactment, and therefore there is nothing to revoke.
Beyond that, in especially far-fetched cases there is room to say that it lapses because of a substantial change in circumstances (see my article on revoking enactments).
Still, there is also a clear and sweeping custom of the entire public, and that too has standing. So for now I am not taking action, even though one has to grit one’s teeth over this invention. I’m waiting for broader agreement first (a significant number that would create a large community of such people).
I cannot say that someone who decides on his own to do this is a wrongdoer. Almost every custom begins with a few people doing it before it is yet a custom, and then the custom is created.

1. Why is it relevant that there is such a view among the medieval authorities? As long as you have no way to rule in accordance with it, it is simply another rejected opinion. After all, already in the period of the medieval authorities the calendar was fixed, so the dispute is not whether there is logic in keeping two days (already then there wasn’t), but whether such an enactment existed. And on that question, as far as I know, no new information has emerged since the medieval authorities.

2. There is no contradiction between what you wrote, correctly, that every custom (or abolition of a custom) begins with a minority, and the fact that those same few are wrongdoers. They are indeed fence-breakers, but if the fence-breakers become the majority among those who observe the commandments, then de facto there is no longer any custom and nothing to preserve.

Rejected by whom? After the Talmud there is no authorized institution that can reject opinions. It is no accident that halakhic decisors rely on opinions they consider rejected in pressing circumstances. Beyond that, when there are good reasons one may break through the fence, and I brought many examples of this in my article on revoking enactments. So the early adopters are not always wrongdoers.

You wrote about wisdom and number.
A. What does “number” mean? The number of sages who established it? But the size of the Sanhedrin is fixed. In this case for example, how many originally established the second festival day, such that we would need to exceed them by even one sage?
B. How does one determine superiority in wisdom of a Sanhedrin or a sage? How is it even possible to measure or decide such a thing — who is wiser: Rabbi Akiva, Ravina, Rav Sherira, Rabbi Yosef ibn Migash, Rashba, the Beit Yosef, Rabbi Akiva Eiger, the Chatam Sofer, the Chazon Ish?

Why would a gathering of a few people like me deciding to stop observing the second festival day not count as a “custom”?
That is exactly our custom:
to decide to treat the second festival day as an ordinary weekday in every respect.
And anyone who continues observing the second festival day is from now on violating the “custom.”
?

I’m asking seriously — I want to understand from when and how something becomes a custom,
and why a custom to behave differently from the existing custom is not itself also a valid custom under the general rules of custom.

Anonymous,
A. Maimonides himself asked that question and answered it. See the beginning of chapter 2 of the Laws of Rebels. It is not likely that this refers to an exact number. If there is broad consensus among the ordained sages of the generation, they can revoke it.
B. The Sanhedrin itself determines its own superiority and that of its predecessors.

Custom,
you are trying to ask Rabbi Yirmiyah-type questions. Customs are not a mathematical matter, and there are no sharp criteria. When there is a significant public among commandment-observant Jews, and rabbis, etc., that practices something, it becomes a custom.

Rabbi Michi,
as a stringency, would it be appropriate for example to put on tefillin on the second festival day, both in the Land of Israel and abroad?

That is one of the implications. I do not do so, because the Jewish law was uprooted in the past and now only the custom remains not to put them on — a custom that is incorrect in my opinion.