What is the ruling on saying: "So-and-so, the deceased (al-marḥūm)," and "May Allah envelop him in His mercy (taghammadahu Allah bi-raḥmatihi)," and "He has moved on to Allah's mercy (intaqala ila raḥmatillah)"?

Saying "So-and-so, the deceased" or "May Allah cover him with His mercy" is permissible as an expression of optimism and hope, not as a statement of fact. Similarly, "He passed on to the mercy of Allah" is an expression of optimism, not a statement of fact, because asserting this definitively pertains to matters of the unseen. It is not appropriate to say "He passed on to the Most Exalted Companion."