Responses

No, they can see him.
Li came in a bag together with Mimi. So 2 people on a bag. People can see Mimi. But since 41 are there, they were wondering who was the extra cause that meant that there was an imposter/Extra (Mimi) of which they weren't sure.
The mask seems more a promp to hide his face. it also contains Mimi soul. It's like a body. The reason why Mimi soul fan materialize and fight snakes(begginer of the chapter) is because of that mask.
Hope this helps.
They 41 people, does that mean that they can't see mimi? Is his soul still trapped in the mask?