0E800 I see no reason for the site to break in IE because of that. If it doesn't support it, it will simply ignore the rules.
However, I didn't test and don't care at all. That's the forum for a game that is only supported on Chrome and Firefox ?
Content-Security-Policy, why would anything break ? If a user-agent doesn't understand a header, it drops it. The only problem might be if the two headers are set with different values... You will need to check the order in which user-agents read them.
From what I've read it was actually recommended to have both headers to support the most browsers ?