Don't be afraid to speak up
Personal opinion...If you KNOW someone has their child in a seat incorrectly PLEASE tell them. A few people noticed a photo of Ben when we first took him home in his seat and they winced We didn't know until later (luckily for us, my Mom was with Ben in the back seat so she could keep her hands and eyes on him.
I'm also a mom in the position of (according to car seat techs) stuck in a rock and a hard place because our van was retrofitted for the tethers but is missing some for forward facing and the seat belts say not to use. I've had many people at stores tell me my kids are buckled wrong when I know they aren't.