I had to edit down the original question to focusing on one question. I invite you to submit the other one in a separate question because it is quite worthwhile. I think that the fundamental issue that separates Marx from embracing a religious view of the world is Marx's idea that dialectical materialism is the foundational element to all being in the world. Marx believes that it is through the process of dialectical materialism that human identity is revealed and has been the basis for all historical consciousness:
The ideas of the ruling class are in every epoch the ruling ideas, i.e., the class which is the ruling material force of society is at the same time its ruling intellectual force.
For Marx, the unerpinning of economics precludes anything elsem from superseding it. This includes religion. Marx would argue that religion is a construct of the "intellectual force" that is embodied in materialism. For example, religion that essentially preaches passivity and salvation in the afterlife enables those in the position of economic power to continue their exploitation unchecked and unlimited. I think that this becomes where Marx cannot embrace a religious view of human existence. It is through his belief that economics is the basis of all being where he separates himself from a religious view of being in the world.