As a lap baby (unpaid), you definitely don’t need a car seat. Some airlines like Northwest are real pricks about not allowing you to use one even on a flight with lots of empty seats. Whereas Alaska is one of the most baby-friendly airlines we’ve used and will let you use an empty seat for a car seat even though you paid nothing. I’ll reserve a window-aisle combo (leaving an empty middle seat) towards the back of the plane to increase the chance of getting a free empty seat. IF someone does buy the seat between, they ALWAYS want to get out from between mother-child or mother-father passing a baby back and forth.

Without a carseat, you will need to have them in your lap for take-off and landing. They might want you to pick them up during possible turbulance as well. What you propose is no different than the unpaid lap baby who has an empty seat next to them. I can’t imagine the flight attendents caring except during take-off and landing and then, (despite the physics involved), Mom’s arms suffice. (or a car seat)

But I would reconsider your plan. Again, I’ve done this A LOT, and we try hard to BRING the car seat, not leave it or check it. Of course the car seat is safest in a crash, but Quantas has never had a crash and Alaska only loses planes about once every 20 years. More importantly, a child can sleep in a car seat much easier because it is sloped and their head is suported on both sides. And your arms will get TIRED of holding him for 18 hours of flight time. And if he gets into a total screaming, kicking fit about being overtired, jet-lagged, etc, you can strap him and pop in a pacificer. For some kids, being secured down helps at times. Sure, it is one more thing to schlepp through the airport, but you could always use a rental cart for $3 or call for one of those electric carts the old folks use. At 1.5 years old, I’d bring a Baby Bjorn front carrier so you can have him on your chest and leave your two hands free. One for the car seat and one to present tickets, passports, etc.

For your carry-on, use a backpack/rucksack. Baby in front, backpack behind, car seat on one arm, one hand free. I’ve gone through many airports and countries that way. So has my wife. Yeah, it is easier with two adults, but doable as one.

Try hard to save nursing or a bottle for climb-out and, less so, descent. The swallowing helps clear his ear pressure.

Theoretically, they can ask to see a US FAA approval sticker for airplane use on the carseat (they all have it), but any nation’s approval ought to fly. And Alaska doesn’t check all that often.

You’ll need a car seat to drive with him in Alaska until he is 4 years or 40 pounds. And while airplanes rarely hit moose, cars and taxis do.

But, their rule contemplated an actual infant, and I think your child would be considered a toddler. You should be ok.