There's some funk with getting seat assignments IME. Just rode JL9 on AA stock and an N fare, and I wasn't able to check in online, nor get seats, even inside T-24. Couldn't do it on AA nor JL website. Even checking in at YYZ the morning of, the check in counter agent could not touch the seat assignments on the JL9 segment.

My suggestion is to get to the check-in counter or talk to the GA before boarding JL9 to see if they can do anything.

One thing I noticed sitting in Y on the 773 is that there is a box presumably storing the electronics that hangs beneath the seats, and as such the height clearance from floor to seat is less than expected. I don't know if a rollie will ever fit under the chairs. My backpack had to be jammed in to get it to fit. FWIW.