Define container; reasonably, a room could be defined as a container. A container of what? And furthermore, define empty! Knowing when something’s “empty” has all kinds of applications. That and teleporting, even a few inches could be incredible, especially dependent on how often/quickly you could do it. 3 & 7, hands down
If I may recommend it, give Symfonium a try for an app front end. It’s actually stellar. I tried Finamp with the same and wasn’t as jazzed