To the point about seeing if someone has a backpack. The essence of WYSIWYG (What You See Is What You Get) is that if it's there, you will see it.

A backpack may be obscurred by wearing a cloak over it, but if they are wearing one, you will eventually see it, or at least the shape as the cloak falls over it.

As to giving piggy back rides....

The mechanics for riding actuall MOUNTS is pretty solid (AFAIK). Piggy back or riding a character's shoulders might be doable, but I would posit that the devs would have to make sure the collision mechanics, and the ambulation mechanics worked properly, Not to metion to potential of needing to MoCap what it looks like IRL to help them establish that in the animations.

Backpacks will be visible. Whatever you see on a character is what they are wearing/carrying.

I don't know if we will be able to carry other people. It's a good question. I know we won't be able to bind or detain others without a bounty token, so we'll see.

I hope and assume backpacks will be visible on the characters carrying them?

Yes, basically every item that you're carrying will not "vanish" and will be either visible or hidden in your pockets/bags assuming it has enough space. I know you will also be able to carry bags in both your hands, but I'm not sure if carrying/dragging people will be implemented.

I would think so? Especially on mules & other carting animals.