I feel your pain. We did a trip to Italy when dd was 12 months.

I think breaking it up is a good idea, the longer you can have a break the better. On the way over we broke it up in Asia and dd had no jet lag, played on the flight and slept in my arms. On the way back we did the back to back flights and it was a lot more traumatic. She wouldn't settle for sleep, got extreme jet lag and just wouldn't settle. That was also a leave in the day arrive back in the day flight (due to time zones) so when we got back she was due to go to bed but she was still wide awake. They made us sleep on the plane because it was night time in our previous destination.

I suggest pack as many activities as you can for her, break the flight up, travel day time so you can keep the night sleeps semi normal.

Oh the memories....lol