The Hadith of al-Bara' on the Torment of the Grave, and the Ruling on al-Ladd
The questioner asks about the long hadith of al-Bara' on the torment of the grave — this was narrated by the compilers of the Sunan and it is a good (jayyid) hadith. It is adduced as evidence. More than one among the scholars of the classical period deemed it to be sound.
As for the second issue, the questioner asks about (ath-thaghaf) — and what is meant by it? And what is its intended meaning? The hadith [of al-Bara'] is well-known and is among the hadiths whose authenticity is agreed upon. The most accurate thing said about that is that it refers to introducing something into the nose — meaning it may cause a person some obstruction of breathing or the like.
It is done as a form of medical treatment, not as a form of harm. If done as harm it becomes forbidden, if done as treatment it becomes permissible. Not everyone is skilled at this, so one who is not skilled at it is to be prevented from it. That is [illustrated by] the Prophet ﷺ when some of the Companions thought that he had lost consciousness and administered the ladd [a remedy forced into the mouth or nose]. The Prophet ﷺ became angry and said: "No one here shall escape being treated with ladd except al-'Abbas, for he was not present with you and did not witness it." And the Prophet ﷺ commanded them to be treated with ladd just as he had been treated.