On the E+ issue - CO sort of already has a mechancism for E+, which is called Premium coach setting (which really is not premium, except that it is in the forward part of the cabin).

UA E+ is still in the coach cabin - just the extra leg room. CO allows Elites and full Y passengers to select seats in the premium section of coach. CO could simply add more leg room to those seats and then have premium economy. I believe that is the way UA does it.

I know CO does not want to lose any seats, but overtime they could be losing business to higher fare Y passengers and Elites who mostly travel internationally.