2D Game: texture atlas and low res / hi res assets

Hi! I'm creating the 2D game and have several questions about best approach to resource management.

Firstly, levels are build completely in unity editor using prefabs. Prefabs have textures. Here is the problem. How can I load high or low resolution textures (depends on screen resolution) if my prefabs already contain material with texture on it?
Is it possible to have high resolution resources and skip first MIP level on small resolutions?
Are there any other ways to support different resolutions?

Secondly, I want to use texture atlases. Could you please advice me the most common way of using them with unity? I don't want to use some kind of GUI engines. I need just two things:
1. Ability to create texture atlas and it's JSON (or whatever) description from my resources textures within unity editor 2. Configure object's mesh UV and material to the appropriate values. It is optional, I can do it by myself.

Thank you!