Perhaps you should be looking at the devices that work and those that don't as a starting point. Our iPhone 6s no longer can log into MW but all other apps both release and beta run. None of the other working apps are from gree in case you were wondering.
This feels very very familier to when the last time we all got locked out so hopefully there is someone left in dev that has been there longer then a year as they might remember how it was sorted out last time.... Makes me wonder if it's an incompatibility with the way MW handles multiple player accounts on a single GC account, er wait a sec is it possible that MW is not logging into GC when booted so it fails and freezes since the GC data is not found. And could this also be the root of the server disconnection issues.....
These log in issues started up at the same time that you guys tried to block the airplane mode glitch so perhaps you might want to look at what you changed back then. It seemed that funzio made the app fairly autonomous to reduce server useage but in doing so opened up the airplane mode glitch. By modifying how the app verified it's data it seems this can of worms opened up.
Devices still on IOS 8 are working so far but now my concern is that fixing MW for IOS 9 will break those on IOS 8...