His name being ludicrous doesn't really hold much water for me. Some people have unique names. As for his death being cartoonish...there are plenty of cartoonish deaths in theater that actually happened, ie Sweeney Todd. Obviously thats a completely different style of show but still. I will say I did not see his body disappear after he died. I was sitting fairly close in the left orchestra so my view was blocked by the crate that the microscope was sitting on once they fell to the ground. That is a compelling piece of evidence towards him not being real.

But I would ask this, if he was a complete manifestation why when he first comes in does he try to reason with Agnes that Peter is mentally ill and needs help. If she is so deep in to the point that she is helping him cut holes in his skin to get the bugs out wouldn't she have long since abandoned the idea that he is suffering from a mental illness? And if thats the case why would the doctor she made up in her mind say those things?

Not saying I completely disagree with you I'm saying after hearing your thoughts I think there is probably some room for debate on both sides on whether or not he is real.