No lounge access if AC is in Y like others said.
What will likely happen is AC will reschedule or cancel the MCI flight, and United will have to open up award space on their own flights, and you ll luck out.
I cant think of a worse way to end a long trip than have to fly AC - and in Y - and on a CR2....
Ethiopian's widebodies are all flat beds now, and I would certainly fly them over United metal, especially since that would add another stop