There's a decent chance the 'angel' Joshua is intended to represent Jesus. (1) Right off the bat, it's pretty obvious that Supernatural regularly deviates from Biblical scripture to a significant extent. Most of the inconsistencies you come across can be explained away with that alone.
The show is based on the concept of urban legends, folklore, and various supernatural myths and creatures from around the world. The overarching storyline also delves into biblical and apocryphal lore, creating a rich and diverse universe for the characters to explore.
