As I said in the first post, decals are repainted on the new round event, and client connect event. Unsaved or deleted decals are not repainted. I tested on CS 1.6, so perhaps DoD has a different new round event.
If you can make a bullet hole, spray paint, or grenade blast on the surface, then you can write on it. If not, then you cannot write on it.
Nope, not possible. If you want special characters or text color, then you will need to use custom sprites. Clients will have to download the custom sprites. I prefer to have no downloads for clients.