If you get into a school ED you dont HAVE to go...you are supposed to but I know one person who was admitted during ED and then R1 at another school and went with the other school. You sign an agreement but I dont think Columbia really has any way to enforce it.
As for round selection, I think Apply to your top choices early. You will have plenty of time to craft essays and stuff if you are thinking about it now. The slight improvements you will make application to application arent really enough to overcome the difference in difficulty between the rounds. I suggest starting with the most thorough application first, this gives you the most material to take from. Then do your others and finally return to your first and rework it. Mine evolved almost until I submitted them. I know Montauk suggests Kelloggs essays as the best to start with and I agree...they definitely cover the most ground and give lots of room to write. So if you are applying to K, write those first and then go to others...then go back and rework everything as you get better at crafting your stories.