As I have said in my last post (did you read it?), your couch most likely has to be a PCO and must take damage from the bullets inorder to react to them. This includes any reaction to explosion force.

(also be sure "hasResponsePhysics" is turned on)
You can try making a new tracer and put it on all your weapons. Make it so the couch only takes damage from the tracer. (but very little damage. You can set damageMod to really tiny number, but as long as it is not zero, the couch will respond to the explosion force). You can also make the tracer invisible if you don't want it to be visible to the player.
If the new tracer is set to only damage the couch and other props you want it to, then the explosion force of that tracer won't effect anything else.
Or better yet, give all the standard bullets an explosion material and customize it's damage system to effect only the props. It will be the same result. The explosion force you set will only effect objects set to take damage from the material2 material setting.

Probably don't need to use tracers if you can just give all the normal bullets a special explosion material. You don't have to worry about "seeing the explosion". That is an effect either as an endEffect on the projectile or a effect activated by the material system. So as long as you don't define the effect, the explosion it self will not be visible.
As for it falling through the building mesh, try adjusting the height of the object so that it is not in contact with the building when it spawns. Sometimes if an object spawns right on or partly through a collision mesh, it will simply fall through. It's the same type of collision bug that some players use to do wall glitches to glitch them selves into buildings they shouldn't be in.
