What can I say? I strive for realism, but in this case it seems I'll have to compromise backstage access. Then again, it's important because there will be countless other times when this problem could happen again.
After checking the graphics file numbers I noticed that G1650 (the lost graphic) shared the file name with a custom graphic in the character graphics folder - problem solved .
I didn't realise that you can't just add 1 to the number of the last file in each folder...
Concerning the chairs, my initial aim was to use the set_terrain call to change the light producing chairs to normal ones (thus dimming the lights in the audience). Chairs were the only objects available to keep the center of the room lit. Good idea, but not helpful in my case (unless there is some way to dim the light left behind).
Two reasons my graphics have not been submited to the Louvre:
1. They are poor quality, I did them in 5 minutes on paint.
2. They are incomplete, I would rather submit them when the collection looks half decent.
Thanks for the comments and advice.