I have twins, and have traveled with them by air and car for quite some time back and forth. As soon as they were old enough, we switched to the only FAA approved restraint device alternative to car seats:

http://www.kidsflysafe.com/ The CARES harness is great! We had one for each child and frequently loaned them out to friends with twins, or singletons. Bring the printout in the odd case that the FA has not seen a CARES, they are quite common now. Then we just checked the car seats at check in, and it was MUCH easier, ours are the type that fold into a storage bag but we've checked plain old boosters too, they've never been damaged, maybe we have just been lucky??

I vote for the night flight, it may take them more time to fall asleep, but they should rest. Take some familiar night time items to help them transition. Personally, I had one whose ears did have trouble with the pressure and it DID help her to suck on something during ascent or descent briefly. One had no trouble at all, oddly, the one who was prone to ear infections had no problem with the pressure.

We brought lots of snacks and little toys that the kids wouldn't normally eat or play with, not sure you would need this on an overnight.

I second the well child visit to be sure the ears are clear, etc. Also, I have had a pediatrician recommend a mild dose of antihistamine for motion sickness and to clear things up. Ask your pediatrician what they recommend, we don't generally do that, depends on if they have had allergies recently, etc.

Also, you can have your child visit the airport, or watch videos on flying, I'm sure there are tons, to familiarize them with what to expect. Many airports have family lines for security, find out if yours does. Take advantage of preboard, boarding when the plane is empty and people are not shoving and pushing you and you have time to acclimate your children has been helpful to us.

Best wishes for a safe and relatively enjoyable trip! Kudos to you for doing such extensive preparation.^