Who prophesied that Jesus would be a high priest after the order of Melchizedek?

King David predicted this in Psalm 110. Jesus being a descendant of Judah could not be a Levitical High Priest like Aaron. Levitical priests were descendants of Levi.

Source: Psalm 110:4

Which Old Testament prophet predicted Jesus coming in the clouds?

Daniel predicted Jesus coming in the clouds. He also predicted his reign over all peoples

Source: Daniel 7:13-14

Who first predicted Christ as King in Numbers?

Balaam the son of Beor was the one who made this prophecy.

Source: Numbers 24:17

The Ethiopian eunuch was reading this prophet when Phillip approached. Which prophet was he reading?

He was reading the prophet Isiah (Esaias) when Phillip approached.

Source: Acts 8:30

What is widely considered to the be earliest promise of Jesus? Hint it is in Genesis

It is Genesis where God says a descendant of Eve will bruise his head. This was right after the fall of man. God promised a Savior.

Source: Genesis 3:14-15