Storage of other speakers in the room, shorting them while not in use? Myth or true?

You don't want anything passively vibrating in response to music signals, whether that's musical instruments or unconnnected speakers. If other speakers have to be in the room, the simplest thing is just to put them face down on the floor or put them with the drive units face to face and off axis to the main speakers.

On the subject of a speaker being a microphone in reverse, lots of studios have Yamaha NS10 or similar woofers converted to act a kickdrum mics.

Wow, this is news to me!

So taking this one step further, What if you have other speakers in your listening room that you use for other purposes? Like having separate components for your AV system and your Audiophile stereo in the same room. The only way around this is to have separate rooms for every use. Some people can afford that, but not the Common man. If you demand the ultimate in sound yes spend a Million makers on a specialized ROOM built onto your home just for listening to music and get over it. The amount of room resonance that will be affected by a speaker is so negligible that you would be much better off doing more with another Room tuning.

Listen naked.... flab might jiggle. Leave the robe on. Although if you listen at night, you neighbors will start a fund me to buy you heavy acoustic curtains. 

Make sure to cover your sink drains, too, or else you might get some inadvertent Helmholtz resonator action.