Your concern is appreciated, although I think you've misunderstood the article. That is not a specific issue, like stagefright. It's just a type of vulnerability that could potentially exist on any device and any platform when a new OS is released. It's very unlikely that this is related to your problems as KitKat has been out long enough for problems of this nature to have been resolved long ago.

Your passwords being changed is odd, but that could have happened from outside your device, and in my opinion that's more likely. The force closing apps seems like running out of memory, and unless you have vpn and tethering(tor?) service on your account, those could be getting reset by your carrier.

 

if your worried about someone gaining root access disable debugging and remove all su permissions for adb shell and terminal. also dont leave your wifi on all the time. anytime any of the radios (bluetooth,nfc) are online its theoretically possible anyone can snoop with enough effort. set a pin code also quit downloading sketchy files. also its almost always someone you know messing with your stuff. if you have a kid maybe its his way of messing with you. o yea make sure you connect to a trusted VPN you do realize when you use a vpn you just connected to a bunch of random computers?