UP Musings: Car seats

We need a new car seat for one of our cars because it is just about outgrown. Both girls are currently in 5 pt harnessed car seats. Three of our seats go to 49" and 65 pounds. But to avoid having to transfer car seats on a daily basis, we have a total of 4 seats (2 per car). We are on the road on long trips a lot, and safety is very important to me. When I started using our older son's Subaru Forester after he left for college, it didn't have a tether anchor in it. The dealership up here had never ordered one, and was clueless. I persisted, and they eventually found the tether bolt and ordered it for us. When Wood took the car in to have it installed, the tech goes "wife a car seat fanatic, huh?".

I'm the only parent in G's kindergarten class that has an issue with them transporting the kids on field trips in parents cars using only seat belts. These are small kindergarten kids - nowhere near the 4'9" tall that kids need to be to use a regular lap/shoulder belt without a booster seat. And they are using the middle, lap only belts on some parents cars, which are totally insufficient in the case of even a low speed accident for small children. I told them at the beginning of the school year that I would have to go on all field trips because G needs to be in a car seat due to her size and weight. Their suggestion was that on field trips, we leave the car seat to be installed in the driving parents car. Over 80% of all car seats are installed incorrectly. The parent would need to be familiar with their owner's manual's instructions for installing a car seat, and my particular seat's installation instructions. It take a good 15 minutes to correctly install one of the kids' seats. All car seats don't fit all cars. Do they really think it's likely that the seat would be properly installed in this scenario?

G is 5 and a half. I started looking at boosters, because I'm not sure how much longer we'll get out of another carseat, even though I prefer the 5 pt harnessed car seats for her. But maybe for the less frequently used seat it would be ok. During my research, I came across a LOT of data indicating that 5 pt seats are the only way to go until they completely outgrow them. Then I saw this: