The twins question is easy. You're only allowed one infant per person. So, if you have twins you and the other adult both have a bag for food, changing mat, wipes, diapers, etc.
If you're already thinking it's too much to spend a few extra bucks to improve the experience for everyone, you probably shouldn't be flying anyway.
I agree on all of your points. However, if there are other factors involed (as I stated, if my child was more difficult), I simply wouldn't travel. Maybe that's not a decision you can make, but I have.
My experience has always been that most kids travel well especially if the parents have prepared.
We sat down and talked about it just a few steps outside. Somebody called the police on us because an officer showed up; the three of us had a good conversation and he left.
How many nappies do you think the average bundle of joy gets through in an almost 24 hour period? You also need their baby food, a changing matt, baby wipes, nappy rash cream, clothing, books and toys to keep them entertained.
Last time I travelled there was hardly room for a small bag under the seat in front, there's no way 24 hours of supplies for a baby would fit easily. Now what if you have twins?