The plane began the dive when the pilot was outside the cockpit. If there was an instrument malfunction and the plane was out of control, the last thing he's going to do is take his hands off the steering wheel (or whatever it is)and leave his seat to go back and unlock the door.........
Not saying a malfunction was the case but that would explain why the door remained locked.
Remember the descent was over a period of between 8 to 18 minutes (depending on where you get the information from). That's a long time even to fight a brain-damaged airplane.
If this was a legit fight with the plane, you'd think he'd want a 6,000 hour captain in the cockpit with him fighting it - and it's only three feet from the right seat to the door.
There was no shout, "It's in a dive!" or "Get the %$^&*&* in here and help me!". Nothing recorded but the captain banging on the door... or so we're told.
The plane did not begin a "dive". By all accounts, it has been referred to as a "controlled descent". This means the nose was up, not down.