Assalaamu alaykum. I want to ask two questions about the following hadith:
"Whoever calls his Muslim brother O Kaafir (disbeliever), then if he is not Kaafir it rebounces back to the one who said it."
I have found different interpretations regarding this hadith by different reliable and renowned scholars of different Schools of Thought:
1. That it is major Kufr (disbelief).
2. It's not Kufr, but it is a major sin.
3. It is Kufr only if the person really means that he is a Kaafir. If the purpose is to insult only, it is not Kufr.
4. If it is said in an interpretable statement, then it is not Kufr.
5. It is only Kufr in case one considers it to be lawful.
6. That this hadith only refers to the Khawarij.
I want to ask: what is the correct and authentic opinion on this hadith, and how is that?
And secondly, if a married woman/man engages in this act harmful act of Takfeer (declaring someone to be a Kaafir), what should they do after having made Takfeer, and how can I advise them?
(Please do not refer me to any fatwa; rather, answer separately, as this will confuse me even more.) Thank you so much.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

The saying of the Prophet : "It would return (bounce back) to one of them," does not mean that the sayer becomes a Kaafir (disbeliever) and goes out of the fold of Islam. Rather, it means that the Takfeer (describing a Muslim as disbeliever) returns to him; that is to say, it is as if he accused himself of Kufr when he accused his Muslim brother of Kufr. Tuhfat-ul-Ahwathi reads, "Him declaring his Muslim brother as a Kaafir returning to him means that what returns is the Takfeer, and not the Kufr itself; it is as if he is accusing himself of Kufr because of him declaring someone else like him as a Kaafir, while that person cannot be declared as a Kaafir except by a non-Muslim who believes that the religion of Islam is false..."

Ibn Hajar said in Fat-h Al-Baari:

"The hadeeth is within the context of deterring the Muslim from saying that to his Muslim brother ... It was also said: the meaning is that his despise of his Muslim brother returned back to him, and the sin of accusing his brother of Kufr is upon him, and this is reasonable. It was also said: It is feared that this would lead him to Kufr, as it is said that sins are a means that leads to Kufr, so it is feared that whoever commits them regularly and insists on committing them, that there would be an evil end to his life [i.e. he would die upon committing them]. The most preponderant opinion is that whoever said this to a person who is known to be a Muslim and there is nothing doubtful about him to justify this claim, then he becomes a Kaafir by that (i.e. it sounds as if he declared himself to be a Kaafir because Kufr must be applied to one of the two)… so the meaning of the hadeeth is: "…his Takfeer would bounce back to him"; what returns is the Takfeer, and not the Kufr; it is as if he declared himself to be a Kaafir because of him declaring someone else like him to be a Kaafir, while that person cannot be declared to be a Kaafir except by a non-Muslim who believes that the religion of Islam is false. This meaning is supported by the other wording in some narrations of the same hadeeth: "…then Kufr must be applied to one of them.

Al-Qurtubi said, "In brief, if the accused person is indeed a Kaafir according to the Shariah, then the sayer has said the truth and the accused bears it. But if the accused is not a Kaafir, then the sin of the accusation bounces back to the accuser. This is the interpretation that could be said about the meaning of ‘bounces back’ and this is one of the most appropriate answers."

It is also interpreted to mean minor Kufr according to some scholars. They provided evidence of the hadeeth narrated by Thaabit ibn Adh-Dhahhaak and reported by Al-Bukhaari. Thaabit narrated that the Prophet said, "Cursing a Muslim is like killing him, and whoever accuses a believer of Kufr, it is like killing him." [Al-Bukhaari]

Killing is not Kufr, and he drew a resemblance between killing and accusing a believer of Kufr. Shaykh Ibn Taymiyyah said in Al-Istiqaamah, "He called him 'brother' when he mentioned him, and he said, 'It falls back on him.' If one of them had completely gone out of the fold of Islam, he would not be called 'brother.'"

Ibn Qudaamah said, "These ahaadeeth were meant to show the seriousness of this act and to deter people from resembling the Kaafirs, and it is not really meant that a person who said so becomes a Kaafir."

It is important to know that this threat applies to a Muslim who declares another Muslim as a Kaafir without misinterpretation or doubt. However, if someone declares another Muslim as a Kaafir with a justifiable misinterpretation and doubt, then the threat that is mentioned in the hadeeth does not apply to him, even if he wrongly judged him to be a Kaafir. When Imaam Al-Bukhaari reported this hadeeth in his Saheeh, he wrote a chapter after it which he entitled: "Scholars who did not view as Kaafir the one who said that with misinterpretation or out of ignorance," and he mentioned the saying of ‘Umar to Haatib ibn Abi Balta’ah, "He is a hypocrite," in it.

Ibn Al-Qayyim said in Zaad Al-Ma‘aad about the story of ‘Umar with Haatib, may Allaah be pleased with both of them:

"If a man attributes to a Muslim hypocrisy or disbelief out of misinterpretation or anger for Allah, His Messenger, and His religion, and not due to following his whims and personal interest, then he does not become a disbeliever by that and is not even sinful for it; rather, he will be rewarded for his intention. This is contrary to the people who follow their whims and desires and the innovators, who declare other people as disbelievers and innovators because of them being different from their whims and ways, while it is more appropriate that the disbelief and innovation apply to them rather than to those whom they considered as Kaafirs and innovators."

Hence, if someone accuses a Muslim of Kufr due to following his whims and desires, then this is the one to whom the blame and the threat are directed in the hadeeth, and he has to repent to Allah sincerely, regret declaring his Muslim brother to be a Kaafir, and be determined not to return to Takfeer again.

As for whoever accuses a Muslim of Kufr due to an existing justifiable doubt that justifies Takfeer, then he is not sinful ,and the blame and the threat that are mentioned in the hadeeth do not apply to him.

Allah knows best.