Or just have a senior member of cabin crew step onto the flight deck whenever one of the flight crew steps out so that there's always two people in the cockpit at any given time, and one can open the door if the other is incapacitated.
So to crash the plane, you only need to get the lead flight attendant under your control. They then kill the cp and your good to go. Ground based unlock? Just get leverage on the IT guy to spoof the signal. Video just gives them a chance to do q selfie diatribe. Life is dangerous. accept it.
Are you thick? The flight attendant wouldn't have a key -- it would be a matter of the flight crew having to physically allow him/her in before one of them leaves the flight deck. The only way this would be any less secure than the current arrangements is if the terrorist attack was an inside job, and cabin crew are no more (or less) corruptible than flight crew, as a general rule.