andreherberth +1 for wordpress bridge, not tight integration. Use a simple SSO that can communicate properties like Flarum Group <--> Wordpress Role, what login process is authoritative (WP is not that good), what User Profile to use (WP is almost never better), etc.
About CMS integration: I have run / designed / written a number of integrated cms / forum / {other weird shtuff} and tight integration is always harder in the end. 38 Special had it right, "Hold on loosely, but don't let go... if hold too tightly, you gonna loose control..."