1. Their bodies are still intact physically, it's hard to explain- Essentially, they're covered in a permanent shadow that absorbs all light that hits it, which is why their colors aren't visible at all. Technically, underneath, they are still intact.
2. The infection CAN spread from simply being touched, but it's much slower than being injured.
3. Jevin is no different than the others, but since he's practically Black's shadow and follows him everywhere (as well as being the one who came up with the prophecy of the darkening in the first place), he was the first one infected. He was fully darkened right away and then sent out to attack the others.