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There are a few volume locker apps on Play which keep ringer volumes at a set level regardless of whether or not the volume button is pressed.
Would that work for you?
Try an app called Slide lock, if you want to try an alternative lock screen.
Not sure if it will be the same, but on my tab running lollipop, if I head to Settings, Sound & Notifications, Interruptions, I can set exclusions so alerts come though when in silent mode etc.
Do you have the same?