Does Christianity support the Genocide of natives, coveting their land, slavery, lynching, imperialism, etc.?

no, and Americans are not all Christians. also, conquer and conflict has always been a part of the world, it is a part of life. this is just the cycle of things. pick up a text book and read for a while, you will find that the area you speak of has always been in conflict. and nothing is rightfully anyones, everything is rightfully everyones and so long as one makes claim to a particular thing there will be conflict.