Hello Midnight Habit,
Sounds like something I did when I was 12.
Steampunk Theremin
You must be connected with the guys in Austin, if not get connected.
"Don't Mess With Texas!"
About your two questions, there will be varying opinions but most successful theremin designers would probably agree.
1: Can I get this to work, with all the metal mass so close below it, simply by tuning?
Yes, as long as anything that might ground out pitch field remains below the theremin circuitry. Think of an imaginary line where above the theremin circuits is the pitch field and below is what I call the "ground field". Others may use a different term.
2: Will a sheet of lead or aluminum foil between the Theremin and feux parts help or just make things worse?
I like this idea if the conducting separation is rigid and mounted firm. Connect a good earth ground to this, 4" below the theremin components is more than adequate. A slight re-tune is most likely.
What you need is time on your side, a project like this does not like deadlines.
Which theremin design are you going to stick on top so everyone in this theremin group can say "Oh not that theremin!". 0-'
Christopher
Edit: Being this is for a video why not construct the theremin art exactly how you want and have someone from this group furnish the sound affect? Avoid true theremin construction all together as it is more difficult than you imagine for reasons unseen! Just have your LED's respond to sound changes.
Sound to Light