Thank you Jason. Hi Stefan,
I am not aware of any changing rooms. None feature on any deck plans so I guess passengers went dressed to play; although given the way the players are featured I think they would have caused a stir and ended up being arrested by the Master-at-Arms if they had walked throught 1st Class dressed as they are drawn.
The skylight is a mystery. There were staterooms immediately above on E-deck, so I cannot see how it could not have gone up and certainly not as far as C-deck. Given that what we have [posted above] is a drawing rather than a photograph, I think it features something that never existed; but hopefully a Titanic structural expert will drop in and answer this one.