Airline rules don’t matter unless the FA’s think they matter. Years ago, after having seats together (3 + 2 — one parent plus 2 small boys, 1 parent plus 1 toddler) for months ahead of time, my family was broken up at the last minute into 5 separate coach seats on a NW flight from MSP to SFO. One of the 5 seats was for a purchased seat for my 18-month-old daughter. No amount of begging the gate agent could remedy this. She insisted the FA would take care of it. The FA would not take care of it. She literally just shook her head and said, “I’m sorry.” Perhaps she was hoping I’d give up the baby’s seat and hold her on my lap, but I told her that I was also sorry, but that obviously I could not supervise, help or be responsible for the toddler from 4 rows back and therefore I expected her to take care of my daughter for 4 hours. Suddenly she was able to rouse herself out of her fugue state to do some rearranging. The lesson I learned was FA’s can do or not do whatever they want regardless of rules.