Yes, these convertible seats are meant to be used until they are outgrown weight wise, their age doesn't matter. I just pulled an extra car seat from my garage, it's a Safety 1st Alpha Omega Elite. The straps are set to one location (the lowest) and I just put my 10 month old in it. He fits the car seat specs and fits him comfortably. Then I put my almost 4 year old, (very petite) daughter in it, without changing anything and she fits in it comfortably too, and her head does not go above the shell. She also fits into the online specs. Mine is a little different b/c you can slide the straps up and down without rethreading them at the top though.
I actually live in Marietta, not far from where this happened, and recently went to the Safety Village fire station where you speak with experts about car seat installation and safety and they actually recommend keeping your child rear facing, up to 2 years, even when their feet are all scrunched up in front of them.....So my point is that it's not really that bizarre that he was in this car seat, as it's quite likely it did fit him (but the placement of those upper straps was definitely wrong.
I wonder though about if the straps were put back to the lowest setting, if they were in fact at a higher setting earlier. That would be very suspicious. I have 3 kids and 5 car seats and rethreading those things is a pain. But I think it's possible that someone could have left it on the lowest setting since they bought the car seat (someone who wasn't the greatest parent), and the straps could have just gone up over his shoulders and back down and not have caused a child great discomfort.
I also think that LH is the mastermind behind all of this. That explains her comment at the jail, questioning whether or not he said too much. And why she referred to RH as the leader of the family at the funeral. She's trying to portray that he's the one in charge, once suspicions became so strong. She's probably building her case now....