Share your experience!
Trusted credentials or Credential manager service? If you haven't tried clearing cache etc from the latter then try that
Thommo is right,for lockscreen go to settings-security and clear credentials.
Do you have encryption enabled?
If you don't have "None" as an option for the screenlock i suspect you have used the user account functionality in Android. If you use that feature in Lollipop (just trying the guest account is enough) that option will be removed. I'm afraid the only solution i know to get it back is a master reset.
Regarding Throw, have you checked my suggestions in this thread: http://talk.sonymobile.com/t5/FAQ/DLNA/m-p/351822