I completely agree with Aimee M's post. I have a 6-month old son and am going through similar situations right now. I've been very protective of his sleep schedule since the very beginning, having read a few sleep books before his birth. I know the benefits of lots of good quality sleep! I've had to deal with push back from family about not traveling and staying at home, but I know I am doing the best thing for my son by giving him the sleep he needs. I also know this schedule won't last forever!
I'm also working with the 2-hour awake window during the day (it was < 60 minutes for the first 3 months for us!), which can be very hard sometimes. On the days I wonder if I'm being crazy about it all, I look at my happy baby boy and remind myself that he is *not* this happy when he hasn't had good sleep. We ALL benefit when a baby is well rested!
I wouldn't travel to visit family/friends for a 6-hr trip with my son until he gets older. We've *just* started making a ~2 hour trip to visit family overnight on a weekend, and we completely time it around his nap. What has worked for us is getting him in the car about an hour after he gets up for the day so that he plays in the car seat for no more than an hour (I sit in the back with him), then falls asleep for 45-60 minutes. By the time he wakes up we are almost there. We bring his bedtime CD, a crib toy he is used to, and a sound machine and use them to put him down for naps/bedtime at the destination. I stick to his familiar routine as much as possible. When people are holding him and I see he's getting fussy and tired, I don't hesitate to say, "Time for your nap... you'll be a happy boy again when you wake up!" and take him and do our routine. The sound machine works wonders to block out household noise in a place that is unfamiliar to him.
As you've experienced, I also know that my son's sleep in the car is not very restful. I avoid it as much as possible. He sleeps best in his crib. We've learned to invite people over to our house for a BBQ (works with the nice weather now in season!) if they want to see us/him, then we can stick to his schedule and keep him in his crib. It's easier on everyone, and they get to see my son when he's in a good mood! I feel like a better mother when I do this, because I feel like I'm putting my son first.
As other people have said, you know your son better than anyone else on this planet. You clearly are a very aware and good mama who knows her son's needs. Do not let other people questioning you make you question yourself! Your son is a very lucky boy to have you! Keep up the great work, Mama!