The FAA now recommends that all infants travel in car seats when on a plane to protect against turbulence. But the red nose guidelines say not to leave babies in car seats for extended periods of time. We are flying from Australia to Europe with our six month old to visit family, which is a very long flight. How do we think about this? Which is the greater risk? Should we keep our baby on our laps/in the plane basinette or Ina car seat on a separate seat? Thank you for your help with this!

FAA is the USA site, but CASA (Aust Govt site) has the same recommendations.
It’s an interesting request (but understandable) - & expensive due to needing an extra seat on flight.
I guess this also depends on the recommendations from the airline company that you are flying with.

I’m not sure if you could quantify the greatest risk to baby using the safety devices you are suggesting, & Red Nose has no information related to using the three flight options.

Red Nose recommendations about car seat use is related to baby being left (partic sleeping) in chin on chest position for extended periods…..ie remove baby once arriving at destination, rather than leave sleeping in car safety device.
https://rednose.org.au/article/are-there-recommendations-for-car-seat-or-baby-seat-use

Bassinet use is generally not applicable for baby at 6 months of age, as most are rolling by then & would not fit in a bassinet type device.
https://rednose.org.au/article/bassinets