Not sure how one couldn't checkin 24 hours ahead...you can do it on your phone or any device. And a mid b number will always get you other than mid solo seats. There is more seats than the amount of people boarding up to c group. In other words..there has to be more seats open than middle as there is more seats other than those than the number that boarded. Only time you will get stuck with middle seats is c group. Cant happen with a or b. Not enough bodies on the plane vs seats.