Songhai embarked unit defense
Solution 1:
I don't really have an answer for (1) and (2), but I'll list my thoughts anyway in case it can help. If anyone can provide a more complete answer I will delete mine.
The defensive rating is astronomical - 500 if I remember correctly. I'm not sure why that is - the actual rating is much lower, embarked units are pretty susceptible to ranged attacks. I don't know what the actual rating is.
I don't know, I think it doesn't.
The main benefit of the promotion is that ships can no longer move into the tile occupied by the embarked units and instantly destroy it - so the only way an embarked unit gets killed is by ranged attacks.