We flew long haul when my month was 7 months pregnant with a lap infant.

Every family has a different dynamic and way to do things. The lap infant was only with my wife to nurse and I held them the rest of the flight.

Your wife will be most comfortable without the lap infant in the larger seat, ideally bulkhead row with the leg rest. You would probably be most comfortable with the lap infant in a car seat next to you in MCE.

No one really knows your guys parenting style and what your little one will allow to give you an answer on what where the kiddo will be spending most of their time but I would first prioritize the configuration that would have the kiddo spending least time on mom and then larger seat.