Wow, you are one tough cookie to convince! LOL This is one of those times when you look at your wife, nod, and say "Yes, Dear."
But as for the physics and logic behind it....
Minor fender benders are very common, you're right. However, you are very unlikely to be involved in a serious collision from the rear. We were in a fairly serious rear ender, we were the front car in a three car collision. The drunk who hit the middle car was doing between 35-45 mph, so we were hit with about 20 mph or more of force. I'm not sure, I don't know the math on force transference or anything. Or even if it's called that.
<a href="http://www.baz.com/wendy/piper/09210401.jpg">Here's a picture of my truck</a>. A Saturn wagon hit a Jeep which hit us. So a decent amount of damage considering we weren't hit by the primary guy. $3k worth. We were stopped, and let's say we were hit at 20 mph. Piper was RFing in the back. She was unhurt, and Nathan and I were sore the next day. I would assume she was as well. She had been sleeping when it happened, so as relaxed as you can get.
If you're involved in a collision of 20 mph you can assume force will be speed x weight. So Piper was 20 pounds and we were hit at 20 mph. That means her body felt 400 pounds of force toward the back of the car. Easy stuff for her harness to catch.
Frontal collisions, on the other hand, are generally much more violent. In order to have a 20 mph frontal collision you either hit something at 20 mph that's stopped, or you and a car hit that are both doing 10 mph. Or 5 and 15, etc. That's unlikely to happen, normally frontal collisions are faster, at least combined. More likely you're going to have something in the 30 mph range each, so 60 mph total. 60 mph x 20 pounds is 1200 pounds of force. Your child's carseat can stand that, too.
So why the concern if the harness can catch? You saw the videos. Those are not done on bias (those aren't even American videos, they're European). They are simply based on physics. A body or object will travel toward the point of impact. RFing the entire body tries to go and the seat back catches from the tush to the head. FFing the harness catches from hips to shoulders. That leaves the head to travel toward the point of impact. The head of a one year old is roughly what, a quarter of their body weight? So again, let's say you have a 20 pound kid. That head weighs 5 pounds alone. In a rear ender that head at 20 mph will weigh 100 pounds. In the frontal example that'll weigh 300 pounds. So you want to put as much protection behind it as possible. It will ramp up the seat's back, but not over, and the entire back will take the force of the collision, spread over the carseat's frame. FFing the head has three times more force and has nothing to hold it back.
Let's see, what else? The neck bones.
http://www.windsorpeak.com/dc/dcboard.php?az=show_mesg&forum=13&topic_id=44503&mesg_id=44503&page shows pictures of the neck bones of one and six year olds. You can see that the one year old's bones are still in three pieces. They are held together by cartilage. They are no where near as strong as a six year old's, and even those aren't as strong as an adult's. They get to full strength about 12 years old. At one year old those three bones can move up to two inches. The spinal column can move up to a half inch. What can happen is called internal decapitation. The spinal cord is severed by the kid's own vertebrae. That is less of a risk as the child gets closer to six.
Someone here has a great analogy. Imaging taking a twizzler and a phone cord, one of the curly ones. Put the twizzler in the middle of the phone cord so it's wrapped around the twizzler. Put your hands on the ends of the twizzler and pull hard. The phone cord will simply straighten, but the twizzler wil snap. That's like the bones and the spinal cord.
A horrible rear ender is definitely something to be concerned about, not even by just RFers. Something that bad is likely to end up in the back seat where the kid is sitting anyway. Some crashes are unsurvivable, no matter how well protected you are. But we play the statistics game here. Statistically you are a lot less likely to be seriously injured in a rear ender than you are in a frontal or lateral collision. You've seen that statistics. 2-4% of serious collisions are rear enders. That's not saying that they're the least common, just that they're the least likely to be serious. The other 96-98% of serious collisions are frontal and lateral (lateral are actually the most deadly since there is less car between the other car and you).
If some guy came and plowed into me from behind while I was stopped and he was going 60 mph, would I rather have my child FFing all the time to prevent that? No. Because if I ram into someone and we're both going 60, that's a lot worse. And that's a much more likely possibility than someone completely forgetting to stop.
As for the LATCH and seatbelt, and the ratcheting, like the others have said you have just turned your child into a crash test dummy. No one knows what would happen.
A carseat is designed to stretch and bend a bit in a collision. It's called ride down, as the poster said above about catching an egg. Imagine that you were going to jump back first onto a bed. You want something with a bit of give, right? If you jumped back onto a piece of plywood you're going to be hurt. So you want the carseat to move a bit, bend a bit, flex, do whatever it's designed to do. By using LATCH and the seatbelt you may have just held down the carseat so much that now it can't work properly, it's not going to give. You've turned your sealy posturepedic into a sheet of plywood.
On top of that, LATCH or the seatbelt is going to stretch up to 20%, again to help with the ride down. Same idea as before, which would you rather have, your seatbelt as it is, or your seatbelt frozen stiff with liquid nitrogen? (assuming no injury from the liquid nitrogen) Your seatbelt will stretch and help you slow down. A seatbelt covered in liquid nitrogen is going to shatter at the same amount of force. It'll do no good then.
So worse case situation for your rigged seats is that they can't do at all what they do and they rip away from the seat and eject your child (because the seat isn't being used properly). Best case situation, everything works beautifully. Let's say it's a 50/50 chance. Play the statistics game now. Is that 50% enough for you to put your child's life on the line? Whereas if you follow the directions properly, use either the LATCH or the seatbelt in the one car, and the seatbelt in the other, then you're looking at 100% that the seat will work properly and your child will be restrained. It has nothing to do with your MacGyver abilities. Save those for the closet shelves, the washer repair, and unlocking the front door without a key. Your carseats are designed to work in one way and one way only. Beyond that no one knows.
Would you take your driver's side seatbelt, the passenger's side seatbelt, and tie them together over your belly and call yourself restrained? Why would you install your child's seat the same way, by second guessing the manufacturer and by making something up?
HTH (remember, "Yes, Dear." She's right on this one.
)
Wendy
