Trust me, this will be the best investment you make. My son didn't like sitting in our laps all the time when we didn't buy another seat. If we were in flight when it was his bed time, he would sleep about 6-7 hours straight unless something loud happened in the plane. But sometimes we would wake him up early to try to get him used to the time change. It depended how crabby he was before he fell asleep as to whether we did this.
Entertainment is obviously important.
For the most part, the flight attendants were nice and helpful. But you will always run into one that either doesn't like kids or is having a bad day and may make it hard to get anything out of the ordinary service. No question, I would stay overnight in Zurich. By the time you get there everyone will probably be tired and need a change of scenery. Our son always did much better if we could stay overnight.