I think that's why Christianity is so successful - people adopted it's faith, while retaining native traditions that are compatible with Christianity. Think about it - why is Christmas close to the Winter solstice? Why is All Hallows set in the traditional harvest season? Here in Ireland we burn bonfires on the Eve of St. John, which is the 23rd of June - near the Summer solstice!!
Christianity laid the foundations of Western Civilization - freedom tempered with morality!
Mixing of religions was not something that was ever encouraged by Christians all the way back to St. Paul...If anything, the evidence of the little mixing there was (similar feast dates etc that you mention) were just a small bone the pagans got out of it. IMO, the progressive 'idea of mixing' is not something to be encouraged now or in the future. And if there is a pure form of litergy/practice that already exists in places like Africa and India, we should protect it from external attempts to gain control and influence in our churches/religion.