Cap'n Kobold
Hero
Pretty sure it already is. Faith can grant power. You can be a cleric that follows almost anything as long as your faith is genuine, or even simply being able to tap into a close enough source of power.My hope is that the Eberron setting will be agnostic.
Fundamentally subjective in ESSENCE.
Bear in mind that while the Sovereigns/dark six aren't the source of power for those religions that you mention, their existence is still believed in and acknowledged by those following the silver flame/blood of vol/undying court.In addition to polytheistic religious systems, Eberron also includes animistic religious systems (ancestor worship of the elves, and cult of the blood of vol). Technically, the Silver Flame is probably better described as monism, but can be characterized as monotheism.
But what makes Eberron Eberron is the uncertainty and subjectivity. Each of these cultural perspectives are equally valid and utilitarian.
Indeed all of those religions exist or function specifically because of aspects of the 'gods'.
Look at it the other way around: Eberron being part of the multiverse means that faith, not the existence of the god that you worship is what grants power. Maybe Eberron is walled off, not by its creators, but by the gods of the rest of the multiverse to prevent such a dangerous idea from getting out.To connect Eberron to the 5e multiverse would destroy the uncertainty, thus destroy the cultural pluralism, thus destroy the mystique of Eberron.