I have one whacky theory about the man in Escanor's Poster. How about that Escanor is Mael's reincarnation. Hear me out. The man in the poster could be Mael and he does look like Escanor in day form. Probably Mael was the original Master of the Sun, some reason died and reincarnated into a lineage that leads to Escanor. That could explain why Nakaba Sensei did not introduce Mael. Plus he did state that Mael was the strongest in the Holy War, and there is only one person besides the Demon King that could defeat Meliodas who was regarded as the most terrifying of Demons, and that was Escanor.
But it's just a theory that I just thought right now.
Reincarnation is possible in Taizai. Look at Oslo and Hawk. They used to be Rou and Wandle.