Well heres a couple of ideas.
1) If this was done by pressing the button it would be done via contraption as the leader of the seance must be really familar with contraptions as he created the illusion of Kira talking to them.
2) There is also another way the culprit could have escaped (through the door) Using a string they could have locked it there.
3) Two people did not show up at the murder location
4) Camerman came late and he came from around the corner while everyones beds where upstairs
5) We cannot forget a main fact of some cases. Some cases the tricks are done relating to their job position. Ex: The doctor would know where the best place to kill Shouko would be.
6) after the seance they claimed someone switched the tapes, but yet the candle exploded.
7) Big fact. Notice how the paintings are toppled over (kinda like dominos) if Kira was responsible for this then why are the candles not knocked over. Therefore the paintings hold some meaning to the murder
8) the speakers hold some part in it as well
9) When the two people are talking they may not be reffering to Shouko but to the other female.
10) Conan hint is : Poster. Could relate to the paintings.
My theory of how it was done