Do the weapons work for added on aircraft?
Yes. You can add them to addon aircraft/rotorcraft (payware and freeware). I use it on a few of my payware aircraft. Getting the 3D Hud working in addon aircraft can be done, but the information for that is is in the Manual and SDK and you'll get a lot of help in the forum here.

Is everything destructable in the game?
No. At least not yet. There is a Topic in the forum which discusses what other things we want Weapon for FSX to be capable of. Captain Sim have said that if there is enough interest shown, additional capabilities can be added. At the moment, AI aircraft, and most boats are destructible. You can't drop bombs yet, a lot of us want them, so hopefully the next update will include that capability. Some users are creating there own mods (including guided bombs) for Weapon for FSX and they can be uploaded here and are available for all to use.

I hope I have provided enough information for you. If not, download the manual and SDK from HERE to read up on it.

Mark