You might want to check into how you came up with 12 - 15 hours if you fly straight through. My dh and I went to HI for our honeymoon. We had a quick flight to Atlanta. From Atlanta to Hawaii it was only about 8 hours.
At 8 hours, the trip was really long. I couldn't imagine doing it with an infant in my lap. If I were traveling with an infant I'd definately try to break the trip up.
The only thing is I think a 3 - 5 hour layover is too much. If I remember correctly from HI to LAX it's only 4 hours. So by skipping the layover you can be there.
I think the ideal situation would be from here to LAX. Have a 1 - 2 hour layover there. Then on to HI.
Somebody on another board once mentioned that they always ask for a seat that has an empty seat beside it. That way if there's nobody next to you that's extra room for you and the baby.
So I would check your flight schedule options and go from there. I've flown when my kids were infants and found they both went to sleep on flights pretty easily. I swear my sons eyes close as the plane lifts, lol.
Have fun!
L.