To what extent is your statement in Fatwa no. 47061 to the questioner, "Try this prayer and this supplication," permissible? And does that align with certainty in what the Messenger of Allah, may Allah's peace and blessings be upon him, said?

The meaning of "try it" is to engage in prayer and supplication for the benefit they contain, not to undertake an act of worship established by Allah's law while doubting its benefit. The Salaf used this expression, and among that is the statement of Jabir concerning the hadith (definition), "Whoever expands (spends generously) upon his family on the day of Ashura, Allah will expand upon him the entire year": "I tried it for forty years, and it never failed." Therefore, there is no harm in using this term with this meaning.