Meh. That makes some sense, but you draw quite bold conclusions from slight evidence. The deities aren't explained in these terms in F&A, F&P or FRCS. FR sources stress extreme polytheism at every turn. Temples are occasionally shared between faiths. There's no reason you couldn't even be a cleric of several gods at once.

You also base a lot of that on the assumption that there's a reserve of divine power that is depleted when answering prayers and charged by souls of the dead. But isn't it reasonable to assume that every act of worship in life, even a short prayer, gives divine points? So encouraging polytheism would actually be in the gods' best interests, since it would multiply the potential worship they get. And moreover, isn't an answered prayer in itself a testimony to the god's power and at the very heart of the divine experience? Why would a successful act of divine-mortal exchange (prayer and answer) deplete a god's power, rather than increase it? There are more questions like this that might qualify your interpretation. I do think your observation about the lack of cohesion and loyalty between the deities of the Faerûnian pantheon (as opposed to, say, the Seldarine) is a good point. In some cases, like in the struggle for the souls of Khem, the pantheon would act in unison against another pantheon. But more often than not, they're happy to let each other mind their own business.

And yeah, while the general rule about patrons and afterlife (though I would question it in many individual cases) is the reality of the setting, there's no reason to make it the centerpiece of our interpretation. It might as well be a quirk, a footnote. You could say it informs the entirety of Faerûnian spirituality and divine relationships, but that's an arbitrary emphasis which the sources do not necessitate. But whatever; we've all read the books, so it would be interesting to see what feel others get from them.

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