No, a scalding hot bath would normally cause burns, I would say. Maybe if it's clear in context that no one gets hurt, it's just an exaggeration for vivid effect, but sci-fi and fantasy books aren't always as carefully written and edited as better literature.
So, no, I'm not aware of any AE/BE difference, and yes, I too agree with #5.
Scalding hot is usually negative, implying a risk of burning one's skin, typically associated with hot liquid. I suppose something like a bath or a cup of tea could be scalding hot, but then you would need to wait a few minutes to get in or take a sip. Scalding is, or was, also a kitchen technique; I think that before milk was pasteurized, for example, it needed to be scalded (but don't quote me on that, I haven't actually looked it up).
Piping hot is positive, typically associated with food brought nice and hot to the table. I picture a dish or plate where you can see visible steam rising.