Unfortunately publishing to Linux is a brand new feature to Unity3D, one we have not yet had the time to explore. Moving forward though we'll be keeping Linux builds running alongside Windows and Mac, have no fear!
Yes! Once we get closer to release we'll be submitting to Steam directly and also making a page on Greenlight. Until that time, you can follow our Greenlight concept page here:
Don't see the answer to your question? Ask the project creator directly.