Thank you for your help CPH-flyer.

You are sure I can do this check in on this JAL PNR 72 hours before? Since I always fly by Finnair all the way usually I never had this problem...

Finnair themselves couldn't give me a clear answer. I am a bit afraid that when I eventually check in for the Finnair flight, something might happen to the JAL seating, like being annulated or auto-assigned.

I don't mind sitting in the middle in case it goes all wrong but I aint the type of person sitting straight for 10 hours, I'd rather turn out to be that annoying passenger waking the sleeping passenger in the aisle seat up all the time..