Does my theory about Christianity / Paganism sound plausible? -Can you confirm & / or add details of your own?

I as a Pagan can confirm several of the things you said. Yes the veneration of Mary comes from Paganism. Yes the Trinity comes from a view of God exclusive to specific forms of Pagan (IE:Hinduism and Egyptian) called Monism. Easter comes from the Spring Equinox, while Christmas from the Winter Solestice. Easter eggs is Roman in origin, Yule trees are Norse in origin.