On our 3 hour flight to Texas my baby sat in my lap and slept the whole time on me. I think I would have been more comfortable getting going an extra seat if we did a longer flight though. He probably would be squirmy and we would need more space
Personally, I have not flown with my 16 month old but I have an idea what I might bring if I were you.. definitely that neon masking tape for him to peel/ play with. (look up flight games w/ toddler on tik tok) Maybe some sort of calming lotion to put on his feet. Plus, tylenol does this too and doubles as a pain relief for teeth. Tons of snack options (salty, sweet, & crunchy). We don’t do screen time but I’d give it up for a 12 hour flight lol. Make friends with the flight attendants. Know their names and tell them you’re traveling alone and a bit nervous/ overwhelmed. Usually they will check on you ( my sister’s a flight attendant and ik she would make sure you guys were good).
Not sure what the answer is, but we had a 4 hour flight right around a year old, and my son did not sit still, and struggled to fall asleep (he eventually did sleep about an hour each flight). We didn’t have the bassinet option. I don’t know what I would have done if my husband wasn’t there to trade off every now and then. I can’t imagine holding him for 12 hours, but at least with the bassinet you have somewhere to put him down, even if he doesn’t sleep. That said, not sure I’d want to lug a car seat thru the airport myself, or worry about much other stuff. Is it an option to pay for his own seat? Not sure if they are allowed in the seat without a car seat at that age, but at least maybe it would give you more space for him to move around, give you a break from holding him, etc.