This is important advice. I had a similar thought when it comes to the advice to stop every time a child cries in the car. A stressed out parent pulling over frequently seems like it could be a dangerous situation.
OP, when your have done everything you can think of to limit time in the car while keeping a sense of balance in your family life and you have tried everything you can think of to find ways to sooth your child you are looking at two options: try to remain calm in the hopes that your driving is safer and your calmness rubs off on your children or you can pull over. I think both of these choices are good enough but they are very personal and have to take into account a lot of personal factors.
For me, my child who hated the car would NOT have done better with me pulling over. It would have just postponed the inevitable and would actually have been a fairly cruel thing to do to her. I never did manage to relax when my first child was crying in the car but I wish I had gotten that advice from someone I trusted because I think it would have been better for both of us if we could have managed it. My second child does not get car sick so she just "regular old" hates the car. I do think when all else fails my relaxed state helps her.
OP, how is your pre-school aged child with the baby? Can you move their seats side by side so they can interact?