1 - Can a true belief be kefirah?
I think the answer is both yes and no.
Yes: Every religion has some core set of beliefs to it. You likely cannot be a Christian if you think Jesus was a mamzer. You cannot be a Muslim if you believe that Mohammad was an illiterate delusional alien from the planet Krypton while Jesus is really God. You cannot be an Orthodox Jew if you believe that God does not exist, but that Baal Peor is the true God.
This is simply definitional.
And if Judaism indeed holds belief X to be a required belief, then if one believes NOT X can be considered a kofer, a denier of a fundamental belief.
And if some subgroup within Judaism holds belief Y to be a required belief, then within that subgroup, one who believes NOT Y can be considered a kofer, a denier of a fundamental belief.
And this has nothing to do with the ultimate truth or falsity of the belief.
Of course, if the religion incorporates a false belief as a required belief, then it is a false religion. But by the standards of that religion, of course one can be a kofer, or a heretic.
No: Religion is supposed to be truth. If the truth is not X, then regardless of what present practitioners of the religion maintain, when the person is judged in the world to come, he cannot be held accountable for maintaining a true belief. So stepping out of our present ikkarim, a Jewish person would not maintain that someone is really a heretic for maintaining a true belief.
Yes: Just to argue the opposite position. Perhaps objective reality does not matter. In terms of halacha, it does not always matter. Consider Rabbi Yehoshua who appeared before Rabban Gamliel with his purse and walking staff on the day he held was Yom Kippur. And consider the tannur shel achnai, where objective truth does not matter. And a זקן ממרה is in violation, in terms of halacha, for going against the Sanhedrin, presumably even if he is in fact historically correct. Perhaps our job is to tie ourselves into the Jewish destiny, and one can actually pasken required beliefs, and over the course of history, new beliefs can be added to those required beliefs. Then, perhaps, the objective truth does not matter, but rather fealty to the consensus which has developed is important. And the Beis Din shel maalah will rule in accordance with the Beis Din shel matta, not by changing reality, but by judging that the person has committed a sin of heresy and is deserving of punishment, even though the truth was with him.
Yes: Just to continue arguing the opposite position. Perhaps objective reality is determined by consensus, just like pesak. There is this idea that a Beis Din's establishing of the chodesh can determine physical reality. (Such as the case of the hymen of three year old plus one day; or whether an infant has developed for a sufficient amount of months to live.) We see people say that the Rambam paskened sheidim out of existence. So perhaps the same is true here. Maybe Hashem had a body in accordance with consensus until Rambam decided otherwise, and set a new consensus and standard for heresy. Maybe the rabbonim can pasken history, that Rav Yonasan Eibeshutz was no Sabbatean, and a new universe is created in which this is true, and in which we live. Maybe they pasken that kabbalah is authentic and the Zohar was written by Rashbi, and so he did.
To be clear, I agree with the first Yes and No. I think the last two Yeses are nonsense. Just like the Rambam, I do not think there is such thing as pesak in matters of belief. If it is true, it cannot be heresy. (On the other hand, maybe one can argue that a true belief can be paskened to be an addition to the ikkarei emunah, against the Rambam.) And there are restrictions on just what constitutes זקן ממרה, which does not extend to believing and even limited teaching; and there is in contrast the case of a horaat bet din where a talmid knows that they erred, and a prohibition from relying on what you know is mistaken pesak. And this is in regard to a Sanhedrin, not a consensus of a certain closed-minded segment of frum society. And no, one cannot pasken reality.
With this set up, we can turn to address the next sub-question: Given that true beliefs cannot be kefirah, is it a convincing argument to one who believes NOT X that he must believe X because it is now one of the ikkarei emunah?