While I'm not a proponent of coded messages to flee slavery being meant every time enslaved African Americans sung spirituals, I am a 'believer' in the notion that more often than not for the 19th century African Americans who composed and sang these songs, the Biblical characters and events were symbolical of real life conditions, if not characters.

My sense is that the meaning of these songs can be multi-layered. Therefore a person can be singing "Let My People Go" and be thinking about the Biblical Isrealites as well as yearning for freedom for himself {herself} and his or her family and other people who are enslaved. "Pharoah's army" can refer to both the Egyptian army in Biblical times, and the US slavery system that kept them in bondage.
Imo, a person could sing "Oh Mary Don't You Weep" and be thinking of his or her own sorrows and at the same time be wanting justice to be meted out to the oppressors he or she faces. In that sense, some spirituals, sometimes, can be considered as the first African American protest songs. It is therefore fitting that quite a number of these traditional spirituals were sung with somewhat revised words during the 1960s African American civil rights movement.

This is not to discount the spiritual and emotional benefit that comes from singing these songs. Though my troubles are trivial compared to those of enslaved people, I often sing these songs privately when I'm sad and need to be comforted, and re-energized.

And as you wrote McGrath, I wouldn't be surprised if the "Maries" in the "Mary Don't You Weep"song were "amalgamated or transposed". I've always thought of the song referred to the Mary who is sister of Martha and Lazarus, but I can see how you could think that Mary referred to Miriam, Moses' and Aaron's sister.

In my opinion, part of the power and beauty of these songs is that they can mean different things to different people.