When I used the Etherwave, sometimes it just refused to be tuned, so placing a can, coin or some metal object on the Etherwave case would make things workable. Also attaching tinfoil to the antenna would also adjust the pitch field. Finally I started using a spring antenna that I could stretch or compress depending on the vagaries of the pitch field.
So, objects are passive unless you have one rotating for vibrato - very doable.
To interfere with the theremin, a strong radio broadcast cuts through occasionally but the most reliable external source for messing up your waveforms is another theremin - close by, preferably being played Two humans play and the two theremins produce "enhancements" not unlike a ring mod sound. ?Not all pairings will produce this effect. Arthur and I have theremins that do not interfere, because Arthur.
What you really want are guitar pedals. Effects are controllable and repeatable.
Mel9 - mellotron it. Strings, vocals and sweetness and everything pretty.
WMD Geiger Counter - yeah, that's the ticket for you! that should ugly things up swell.
I use both of those pedals for "National Electrophonic" on youtube.
But you do not have a theremin . . . you don't want a theremin that is easy to play, no one does. I prefer an easy instrument to play that I don't have to use vibrato with and can play staccato as well as legato.
so you buy whatever everyone else buys because I don't relish any competition.
And remember PRACTICE, PRACTICE, PRACTICE
Something I would never do. I just play.