The answers there don’t seem reasonable to me. It usually goes along the lines of: you need to follow the place where the center of your life is. But the reason they celebrate a second Jewish holiday day is because they didn’t know when the new month began, so what relevance does where I usually live have to that?

If you saw an answer and something doesn’t seem right to you, please point it out and formulate the problem.
It indeed goes according to where the center of your life is. The original reason was that they didn’t know when the new month began, but that was relevant only outside the Land of Israel. In Israel they knew. Now it has been established that everything goes according to where the center of your life is. What is the problem with that?
I have a problem with the law itself, even abroad, but that is a different discussion.

My question is whether this rule—that a person who is in a country that practices differently from his own country has to follow his own country—has always existed, or whether it is something they established more recently. And does this also apply to other customs / laws?

In fact, the halakhic decisors disagreed about the law regarding a person from abroad who is staying in Israel (the opposite case is written in the Shulchan Arukh). The common practice is to rule as I wrote to you (following the view of the Mishnah Berurah).
As for the question of “do not form separate factions” (different customs in the same place), that is a very good question. I saw that they cited responsa Avkat Rokhel, section 26, which dealt with it, and Igrot Moshe also quotes it. See a survey here:

צירוף בני ארץ ישראל ובני חו"ל ביום טוב שני

Thank you for the response. How would you recommend that I act?

This whole issue according to the Mishnah Berurah seems very puzzling to me. I can decide that I feel like moving back to live in Israel for a period, and then all the laws relevant to me change. And why not go even further back—I was born in Israel, so why shouldn’t I be considered Israeli for this purpose? Not only that, I also spend a lot of time visiting Israel… This whole thing creates a lot of confusion for me. There was a period when we were in Israel for a few months, but we hadn’t left abroad, and my wife didn’t want to observe the second holiday day at all, and I called rabbis who told me that I had to keep two days. In the end I wound up doing the Passover Seder alone on the second night, and it created a lot of chaos, and we were also staying with Israeli family and they couldn’t eat leavened food near us. The approach of Hakham Tzvi sounds much more reasonable to me from that standpoint.

I have no recommendations. The options are before you, and you are the one who decides.

I’ll just note that the arguments you raised here are not relevant to the issue. In my view, they are simply incorrect.