I've been pondering this for a couple of days, since gree aren't interested in resolving this issue I'll post up some questions to see if we can find a common denominator which is causing the issues.
Though not locked out I see a lot of session expired warnings each time I log into a linked account so my first question is are those locked out using linked accounts? Now the reason I ask this is I'm suspecting that gree has throttled back on the number of available sessions for MW and as a result they have also changed something on the server to boot you much sooner than before. Now that doesn't exactly pinpoint the log in issue if you are already online but I'm wondering if this is connected to one feature which is the push notifications.
Reason I mention this is in the past even if you are not logged into the game IOS devices would constantly pop up messages such as you got attacked. Because I hated that feature I made sure it was disabled. So just wondering if those having issues have this turned on. Easy to check, go to Settings, scroll down to MW click, turn off notifications.
Also have you tried a full restart in case something is 'stuck'?
And in typical gree fashion the write up on the app store in incorrect and the oldest version of IOS that is support is not 5.1.1! In fact I pointed this issue out over a year ago.
From my testing the oldest IOS devices still supported are, iPad 2nd generation, iPhone 4s (though maybe not any longer) and iPod 5th gen. iPhone 6's being locked out is interesting though given how spectacular testing seems to be it's not surprising.
On all my devices using linking each fresh log in generates a null server response followed by a retry connection button. Then typically after a wait the account opens.
On Android it depends on the device, I've given up on anything that isn't quad core and with a minimum of 1 gb or system memory. I still get the same log in errors as IOS though. Oh yeah Android leaders still cannot purchase walls as clicking that button freezes the app.
Bluestacks, tested on OSX 10.8 and it runs though fairly slowly on Core2 Macs and passably on Core i7 with latest release OS. On Windows a C2D is painfully slow with i5's with 8gb ram better and i7's about the same as Mac.
And there is no shame in throwing up your hands and saying screw you gree, no shame at all…..