Questions about Clans

I've seen people who owned the clan successfully request in support for the support team to change their clan name and tags.

You and everyone who posts on the diplomacy board can see the posts made here. It costs gold to talk on it so you have to think twice about spamming it. All the members in your clan get a notification marker showing that you have a new there.
You can block people from posting on the diplomacy board if you're the Chief or VP. You can also remove posts from the board if you're the VP or Chief. The mission statement, the owner and vp can set.

Anyone in your realm who the VP or Chief hasn't blocked from posting on the board can post on the board. Anyone in your realm can view your mission statement and diplomacy board. (If I'm not in your realm, I can't see your clan as of right now.)

You can set the clan to private so that the owner and the vp can accept or reject people who request to join. You can write the join requirements on the Mission Statement so people know to not apply if they don't meet the requirements.