You could either spawn things in, or simply put every thing in the map, then hide all but the one, then unhide I've at a time, after one is shot. This is easier if it's randomized. Then choosing next is simply l indexing the next item in the array.
It should work. I did the same thing with the same song code. Make sure you clip is a script brush, and that its targetname is the same in the GetEnt function.
You could also have it initialize the teleporter script after completing the shootable EE script.
You can remove a clip box by simply making it a clip_player and making it a script brushmodel. use GetEnt to get the targetname of that script model and delete it when you want it gone. Alternatively you could just use TriggerEnable( false ) on your teleport trigger and set it to true after your easter egg is complete