The orthodox belief you state that Jesus is the same as God is basically equivalent to the Hindu belief that their various gods are just different facets or aspects of God (with a capital G). If Christianity is monotheistic then, by this definition, so is Hinduism.
As for veneration of saints and relics, then anything that worshipped is a god and asking for blessings, intervention in worldly affairs, etc are all acts of worship. So when someone prays to a particular saint seeking something they are engaging a polytheistic act just as one who prays to an idol is engaging in polytheism.
I am not sure about Muslims worshipping Muhammad. Maybe in some polytheistic branches of Islam such as sufism and so forth which were themselves influenced by hindu/christian/etc ideas.
As for veneration of saints and relics, then anything that worshipped is a god and asking for blessings, intervention in worldly affairs, etc are all acts of worship. So when someone prays to a particular saint seeking something they are engaging a polytheistic act just as one who prays to an idol is engaging in polytheism.
I am not sure about Muslims worshipping Muhammad. Maybe in some polytheistic branches of Islam such as sufism and so forth which were themselves influenced by hindu/christian/etc ideas.