What is the difference between the imperative and past tense forms in expressions indicating supplication, and do declarative forms also convey the meaning of supplication, and what about His saying, Exalted is He: "Allah was pleased with them, and they with Him"?

The phrase "Allahumma Salli 'ala Muhammad" (O Allah, send prayers upon Muhammad) is a supplication, and its imperative form presents no issue, as a request from a lower to a higher being is a supplication. As for "Sallallahu 'alayhi wa sallam" (may Allah send prayers and peace upon him) and phrases like "Rahimahullah" (may Allah have mercy on him) and "Ghafarallah lahu" (may Allah forgive him), these are statements intended as supplications, which is a well-known rhetorical style. The phrase "Radiyallahu 'anhum" (may Allah be pleased with them) is recommended for the Companions, and it is a supplication. It can also be a statement of fact for those Companions about whom the Qur'an explicitly states Allah's pleasure.