I have traveled a lot with my kids. For what it is worth, here are my opinions. First, about the time transition. To make your long flight as peaceful as possible, I would not transition your child until you arrive in the States. Plus, you will be able to get all those last-minute things done easier with your kids still sleeping in Germany time! My experience is that flight travel is hard on the kids and they will be very sleepy, even if they do sleep on the plane, once you arrive i.e. your child will have jet lag. I have been surprised to find that my kids have settled into the new time zones days before my husband and I have been able to!
As far as the airplane travel (I am also doing this with my 13-month old this summer, but to the West coast), expect the worst and hope for the best. Honestly, you need to set your mind like that. I flew with my oldest from the West coast to the East coast and he screamed for almost five hours--it was terrible...we tried it all and nothing helped. In the end, we figured his ears were hurting, but there was absolutely nothing we could do to settle him down. He didn't settle until we started to descend.
Hopefully your experience (and my experience with my baby) will be much better than that. Bring lots of toys, books, and anything else that might seem interesting. Oh, snacks--lots of snacks! Try to get as much rest yourself as possible the days leading up to your flight. You will need the stamina! Good luck!