Our now 7yo DS has lots of flights under his belt, starting at 3.5 months old. The trips when he was under 2yo were a breeze.
My random tips/thoughts/suggestions -
Get to the airport as early as possible, it cuts down on your stress level if you don't need to rush. If you are driving, factor in the time it will take to park and get the shuttle back to the terminal.
When you check-in at the counter, ask to speak to an agent about the possibility of being seated next to an empty seat. If you lucky, they might do this for you, giving your LO a seat. The chance of this happening may be slim depending on time of day, how full the flight is, etc. but I had great success with this when DS was a baby. In general, I think flights are much fuller but it can't hurt to ask.
Speaking of seating, double check your boarding passes for your seating assignments if being seated together is important. If is it and you were seperated, do not walk away from the ticket counter without getting it corrected. This is situation where having extra time pays off. You cannot count on the FAs to help you with seating arrangements, it isn't part of their job. At best, they might try to find someone willing to switch seats. At worst, they will tell you it isn't part of their job and you will be left begging someone to switch seats.
This happened to us on our last trip. Our confirmation showed us together, we checked in at the airport and didn't bother to look at our boarding passes. Once on the plane, we discovered we were seperated. DS and I were together and DH was a couple of seats away. Because of DS's age, it was ok but had he been younger, it would have been tough. Turns out, the type of jet was changed at the last minute and everyone got new seat assignments. This was a return flight from a popular family destination and people were completely freaking out about being seperated. The head FA got on the PA and basically said "sorry, too bad but we can't help you, take your seats so we can go."
We have had nothing but good experiences with the TSA, every agent has been respectful of our travelling family situation. Sometimes we were even pulled out and taken to an empty line so we didn't have to wait. Some airports have designationed lines for families.
We have a system for security that hasn't failed us yet. One parent takes care of the child and nothing else (except for your own boarding pass) and the other parent is in charge of any carry-on luggage.
So as you approach the screeners, carry-on parent does their thing while baby parent carries the baby through. Both parents move through the screeners, baby parent heads towards the bench area (where everyone will be putting their shoes on) and the carry-on parent collects the bags off of the x-ray belt. There is no reason to wait for each other.
There is also no reason for baby parent to stand around in the fray at the xray belt. Ideally, baby parent trusts carry-on parent to do their job and you all walk away happy. What you don't want happening is for both of you to be standing in the middle of all the other travellers who are trying to get their bags going back and forth about "did you get the diaper bag? who has the carry-on?" etc.
Arriving at the gate early will give your LO time to burn off some energy.
Take whatever food and drinks he will need with you on the plane, even if that means buying it after security. Airlines are stocking less and less on flights. We recently flew first class and couldn't get milk. Not a huge deal for us but an example of simple basics that likely won't be available on the plane.
I forget what 12mos like but I remember stickers occupying my DS for hours on one flight. I was covered, he was covered, the seats were covered but he was happy and quiet. (I cleaned them up prior to landing.) Stickers are probably not for a 12 mo but what about something he can touch, feel and examine?
We always took advantage of boarding early. I have heard some parents like wait until the last minute but for us, boarding asap gave us time to setlle in. Again, this was a stress management tool.
I think there is a fine line between taking too little to keep a LO occupied and too much. I made the too much mistake twice, stuff was ending up on the floor, which I was constantly retrieving.
Take an extra set of clothing for everyone. Knock wood, you won't need it but if you do, you will be so thankful to have it. I pack an outfit for each in large ziploc bags and compress the bags as much as possible to remove all the air. I end up with three flat packages that go in the bottom of the carry-on. They take up very little room. Don't just throw clothing in the bag, that takes up too much room and too much energy when you are rooting around trying to find something else.
(The only time I needed my change of clothing wasn't because of a sick baby, it was when a FA dumpled an entire tray of drinks onto me.)
It isn't a given that all LOs will be bothered by ear pressure but if nursing or letting him have little sips of water makes you feel better, go for it. But if your LO is sleeping or otherwise resting peacefully, I wouldn't distrub them just to nurse during take-offs and landings. When our son was an infant, the drone of the jet engines would have him asleep before take off.
For a long-haul flight, I don't think a layover is a bad thing as it gives you all a chance to stretch your legs. Don't know how that will work at night though. Take a light-weight, dark blanket to drape over LO's head if he is asleep upon landing and deplaning, maybe you can keep him asleep.