Can a Muslim be considered to have committed an act of disbelief (kufr) because of his saying: "How can a Muslim grieve when Allah is his Lord?" or because he rebuked one of his brothers while the latter was carrying a phone containing Quranic verses?

None of what was mentioned necessitates disbelief, for Allah Almighty says: "But whoever opens his breast to disbelief - upon them is wrath from Allah, and for them is a great punishment." And the questioner was wary of disbelief. It is incumbent upon the Mufti (definition) and the judge to act in a way that keeps the Muslim within the fold of Islam, even if there are ninety-nine reasons pointing to his disbelief and only one to keep him within Islam, acting upon the hadith (definition): "Avert the prescribed punishments from the Muslims as much as you can."