My guess right now is that something changed in the setup that Sibelius does not like. Perhaps an OSX update? OSX 10.11.4 is available too, if that matters. Sounds like you may need to uninstall and reinstall Sibelius. But before doing so, try the following: go into the Sib user's folder (/Users/yourusername/Library/Application Support/Avid/Sibelius) and delete the "Playback Configurations" directory. Sibelius will generate a new one upon launch/restart. Create a new Configuration, see how it goes.
4) That image is of File > Preferences > Playback. Playback Configurations are defined at Play > Setup.