I am in the same boat as you.....I took off all the old junk in my new house & want to replace it all. Looking at what they did originally I really had to laugh. In one area, they took one huge grill & cut it in two to cover 2 different holes in the ducting & return. Then I have a little 2 inch by 8 in the wall & can't fine a thing at Lowe's for that. I am going to go internet shopping when I have all the sized I need figured out. It is very entertaining to take apart old things that were thrown together. After I'm done, there isn't going to be an old anything in the house (& that is only 15 years old at that). Things in this house are actually newer than the new house I purchased in California 20 years ago. That's just a mental thing it seems. I just like everything all new & clean & fresh. Always owned new homes, (& even when they are 20 years old, I was the original owner in what is now the older home).....??????

Anyway....it's a lot of work for sure trying to find the right size anything.

But I am also like you......I have theories about air flow also....it seems that return should be horizontal because the air is coming in from the floor direction. Hot air should be aimed down since it rises & cold air should be aimed up to cool off the warm air quicker. But I do have the circular air ducts in the ceiling for both heat & cold....no choice in the direction there & no choice in the floor ducts buy I purchased the fancy victorian floor duct covers with scrolly grill work....the luvers inside I have opening toward the wall so it can bounce off & into the room instead of shooting directly into the room.....just preference there.....but actually thought out about the air flow before placing things.

Just to let you know, you aren't off base in questioning these things,
Debbie