Moghedien hold saidar, when Graendal attacked her & Cyndane with Compulsion in PoD; Graendal still thought she would be successful. It was a very strong attack, though. Maybe at least little Compulsion "tricks" don't affect a channeler who holds the OP. That means it would be more difficult to use Compulsion on a channeler, who holds the OP, but not impossible, if your Compulsion is very strong. Likely the Crystal Throne's effects actually aren't that strong, but it just spreads a general aura of awe, etc. .