Yay so glad you posted this! It seriously bugs the crap out of me when people just do whatever and pay no attention to whats safest. Dh's little brother is maybe 17 months and has been forward facing in an out of date carseart since he was maybe 11 months! And was under 20 pounds!
I agree with the booster seat guidelines. A cousin of mine has their 2 year old in a low back booster with just a seat belt and I think this is very dangerous and he would be hurt in a car accident. We have been talking about the guidelines lately because my mom is needing to switch my nephew from a car seat to a booster so we were discussing what kind he needs, we agree on high back, but I think he still needs the kind with the harness because he is only three and would probably be able to unbuckle himself from a regular seat belt, even being in a booster.
I would definitely say that a 3 year old needs a 5 point harness - for a LONG time to come!
We already do the extended rear facing because it is safer, and use the 5 point harness until the child outgrows the weight/height limits of it.