If the room was in a different place (is this right) why, and why would no one notice?
See answer below about the bath...
What actually happened to people, nasty stuff fell on them (due to pressure on the mattress?) so they had a bath... then what happened to them? And where did they go?
The pressure of bath + water + person made the bath sort of tip end up and dump them in a huge water tank, where it then closed up again and left them to drown. The attic had to be elsewhere to give enough space for the water tank etc. Why no-one noticed? Official line - spiral staircase disorients. Actual line - they is morons.
Was it all set up by the original nazi chap to kill the first guy as JC thought when I went to bed? Was that the only point? Was any of "today's" characters aware? What about the mad mother?
Yup, was him, the others were just accidents. None of them aware, the mad mother rescued painting instead of man in fire... no real point to that either.
And clearly Elodie met a sticky end - why, how, who, does it matter? When I went to bed she had just not been released by a kidnapper who picked up the cash.
Why - she was leaving him for the roadie, throat slit, magician, nope - was a pointless side story.
And was there anything I need to know eg about JC's girlfriend character?
Not really - she sodded off with the guy on the phone in America
Now can someone tell me... what happened with Carla to make her announce he was dead???