Why is the work of "pass over" said to be done by an angel? Exodus and other passages clearly state that it was God Himself who passed over and allowed the plague of death to enter the homes without the blood applied. Are there any passages that verify a "death angel"?

There are no references in the Bible to a death angel. Exodus 12:12, 23 and 29 all explicitly refer to God going through the land, striking down the first born, and passing over the homes with the blood. It is possible that God used angels that night. An angel spoke to Moses in the burning bush. An angel was in the cloud by day and the fire by night. The New Testament speaks of angels speaking to Moses on Sinai, as well as giving the Ten Commandments. In addition there are numerous references to the "Angel of the Lord." However, scholars are divided over whether this angel was God Himself, a particular angel that God used, or even the Lord Jesus. 

The Bible does not say. The idea of the death angel may have developed because people did not want to attribute directly to the hand of God the death of these children. The point is that, though an angel may have carried out the work, it was nonetheless God’s decree, and He Himself is said to have done it.

Michael and Gabriel are the only angels mentioned in the scriptures to our
knowledge.

The ministry of angels is clearly taught in the Bible. "Are they not all ministering spirits sent forth to minister for them who shall be heirs of salvation?" (Hebrews 1:14) Not all of the areas of their activity are indicated but some are and should bring joy . There is an old hymn:

"O come, angel band, come, and around me stand; And bear me away on your snowy wings to my immortal home, to my immortal home!"   Luke 16 speaks of Lazarus "carried by the angels into Abraham's bosom".