Supernatural creatures are magical creatures, mortals use magic. That's an important distinction, I think. Magic is not something these being do, it's just who they are. A bird doesn't think about flying, it just spreads its wings and flies. And since magic is part of everything these creatures do, they will get blocked by the circle that's meant to block magic. If they could reign in their magic, they could break the circle, but that's where free will comes in. They can't help but use magic the same way a bird will spread its wings.
Though like any barrier, a magic circle can be broken down by brute force. Holding something like Toot is easy, he doesn't have much metaphysical strength. Earlking on the other hand could try to break through a circle, that's why Harry is reinforcing his circle the way he does when he summons him.
Inanimate objects don't break a circle on their own volition. But they provide a stepping stone for a creature that might otherwise be affected by the circle. Like rubber gloves to push openan electric fence. They won't open the fence on their own, but you can put them on and are now able to move it without getting shocked.
The analogy works for the air problem as well. Air insulates just as much, it's just not possible to use it as a tool, like the rubber gloves.
Mortals are only using magic, their natural state is mundane. That means they are naturally insulated and don't need anything extra to break a circle. They can use something extra, like the film cannister, but they can simply reach through a circle and it will break, unless it is designed to keep them out.
The shape is largely irrelevant, I would say. It's in the shape you draw on the floor, usually a (rough) circle. Escaping in any direction simply isn't possible, it closes to all sides, top and bottom included. If you need it to be a shape, I would imagine some sort of dome on top, rather than a flat top or some complicated shape.
