I would always say fly direct if you can. I can't imagine it would be much fun to have to make one journey, then get off the plane and wait for the next plane with a baby!
When you check in most airlines ask how old your baby is, rather than the weight...not sure why!
The last long haul flights we took we were v lucky to get an empty seat next to us for our son, he is too big for the bassinet. I don't know how we would of coped without that extra seat, espec. now he is walking and v active....and he's heavy! It would of been very tough to of held him for 11-12 hrs.
I would consider buying him a seat from now on, although we fly standby so wouldn't make any difference, he still wouldn't be guranteed a seat!