1. When did Jacob become a prophet?
Many, many years before any of us were born.
For Redemption purposes he's been prophet for years, probably always but the C/D deck was released before my time.
2. Doesnt Joab say band to a RED hero from Sam.1 or Sam.2?
You're correct that he can't band to Armor Bearer, eliminating red from that banding chain. You could add red like this.
Simeon
Jacob
Seraphim (bands to green)
Armor Bearer
Mahari
Claudia
Mentor
Ethiopian Treasurer