Is it true that Umar ibn al-Khaṭṭāb, may Allah be pleased with him, grabbed the garment of the Messenger of Allah, may Allah bless him and grant him peace, when he forbade him from praying over the hypocrites? And is this act disbelief (kufr)?

The hadith is authentic, and Umar (may Allah be pleased with him) did not intend to disrespect the Prophet (peace be upon him) when he seized his garment. Rather, it was out of zealousness for the Shari'a of Allah and his eagerness to prevent the Prophet from praying over the chief of the hypocrites. His action was not an offense but a result of what was going through his mind. Umar himself acknowledged his astonishment at his own audacity. Reverence and exaltation of the Prophet (peace be upon him) are an obligatory duty, but merely seizing his garment is not an act of disbelief (kufr). The proof of this is that a Bedouin once pulled the Prophet's (peace be upon him) cloak so forcefully that it left a mark on his neck, and that was not considered disbelief.