The real pronunciation of this consonantal construction (if there ever was one) is lost in time.
Etymology or history, if you like, doesn’t determine pronunciation any more than it determines definition. The “real” pronunciation of Jah is exactly the way the Rastas pronounce it.
But in a certain way, if I may engage in a certain conceit, it is only understandable from within or at least in reference to, the Hebrew story of Exodus and Babylonian exile.
A numerologist will tell you that you can only understand the song if you do a numerological analysis. A linguist will tell you that it needs to be understood linguistically. A historian will tell you the real understanding is historical. You think you have the “only” understanding of the song? Get in line.
All art speaks for itself. The meaning isn’t in the song, it’s in you. The song has no meaning until you bring one. Yours is no better or worse than anyone else’s. Even if the artist himself tells you what the work means to him, it is just another opinion. People get from art what they get and there is no right or wrong or true or false or better or worse. Of course, this is all simply my opinion.