As he was now approaching

"As Jesus was going near." Jesus' disciples were traveling with him.

where the Mount of Olives descends

"where the road goes down from the Mount of Olives"

"great things they had seen Jesus do"

They were saying this about Jesus.

Here "name" refers to power and authority. Also, "Lord" refers to God. (See: rc://en/ta/man/translate/figs-metonymy)

"May there be peace in heaven." They are expressing what they want to happen.

"highest" refers to heaven, where God lives. AT: "let everyone give glory to God in the highest heaven" or "let everyone praise God the Highest One"