If the generally accepted theory of earths formations is....?

That is was at one point a molten mass bombarded by space debris... and at some point a space rock the size of mars smashed into the earth... 1. Why dont we have radioactive uniformity? 2. If the age of the earth is measured according to the age of the rocks there in, and those rocks came from elsewhere, how can we conclude the age of the rocks gives us an accurate measurement?.... and this then extends to the fossils there in?