Was the angel who descended upon Mary Gabriel (peace be upon him)? And does his descent upon her mean that she was a prophetess? And were the angels in human form when they gave glad tidings to Sarah?

The angel who descended upon Mary was Gabriel, peace be upon him. This does not necessitate her prophethood, because angels have spoken with people other than prophets. Prophethood is, according to the preponderant opinion, exclusive to men, due to the Almighty's saying: "And We sent not before you except men." Scholars have refuted those who assert the prophethood of women by stating that the mere address of angels does not imply prophethood; thus, Mary and Sarah were not prophetesses. As for the angels who gave glad tidings to Sarah, they appeared in human form, and Sarah did not know their true identity until they revealed it.