> 2. A lot of the rest are compatible
No one religion covers "most people." You could argue that Christianity and Islam (which add up to ~55%) are the same God because of their Abrahamic roots, but both religions have very important disagreements on the true nature of God that are fundamental to their beliefs and fundamentally incompatible with each other.
Their definitions of God do agree that there is exactly one God... which is fundamentally incompatible with the next two biggest religions (Hinduism and Buddhism) that both hold "there are many gods/divine heavenly beings" as core beliefs.