I would make/buy her a busy book. The best way I have seen to do it is to get those clear plastic binder pouches- the kind that have a slider on the end to zip open and closed. Then fill each one up with different busy things... One with beads and strings, one with felt dress-up dolls, one with crayons and paper, one with toy cars, etc. maybe even buy some new small toys to play with in it.
A bag of dum-drums (or other slow-to-eat treats...) will probably help too. ;) lol.
As for the car seat, I would reccomend gate-checking it (where you carry it with you through the airport, and check it when you board he train.) Most airlines do so for free, and that way you don't have to worry about it possibly getting lost or damaged it's regular luggage. Some people like to use their seats on the plane, and if you plan to go that route I would call the airline and ask about your particular seat... Sometimes, even if it SAYS it is airline approved it doesn't work too well, and you wind up having to hate-check it anyway. (It's easier not to deal with it, trying to haul it through the tiny aisles while wrestling with luggage and a toddler... I'm not too worried about the safety aspect either, considering that in the event of a plane crash, the seat is kind of like a drop in the bucket at that point.) I would only check it with the baggage (again, Most airlines do for free) if you will be having a few layovers, and don't want to haul it around multiple airports.
When we flew with our DD, she was almost 2. We actually used her big stroller as a luggage carrier, and carried her. Much easier to maneuver! (They also gate-check strollers for free...)
For food, I would try to find as many healthy snack items as you can. Snacking was a great distraction, and I can't imagine trying to to eat a meal on the plane with a 2yo. (Which, with a 12 hour flight, I'm sure you will have to try...). You won't be able to take drinks through security, but you can definitely take some empty straw/sippy cups (or whatever your dd uses) through empty, then fill them up with water or whatever you buy once past the checkpoint.
Also... If your DD has a lovey, make sure you check for it before leaving and boarding the plane. My daughter managed to lose hers at some point, and was devastated when she had to sleep without it. I didn't even notice it was gone until bedtime. :(