Didn't bring a carrier, so I can't comment on that. But I can say the TSA were absolute jerks when I was flying back. Made me take my sleeping baby out of the car seat (was expecting that, so ok), but they were super rude that I was "holding up the line", and once I had the baby in my arms they asked me to put my pram and car seat on the x-ray belt, which I could see was not going to fit. Different agents kept telling me to do different, conflicting things to do in a span of about two minutes. I was so confused and I was trying to hurry that I accidentally sliced my finger and it was bleeding everywhere while trying to wrangle the baby, the stroller, car seat, carry-on stuff, shoes, coat, etc. It was a real hoot.
As compared to Heathrow, they let me push my kid in the car seat and stroller through the x-ray machine and then just patted it down and ran down it with the wand. They also actually helped me move my stuff along. It was like a night and day difference. The US bugs me sometimes.