that Lubitz studied the door locking system days before on his home computer and this was a major part of his plan to carry out this mass murder undisturbed
If you read the original text about that it just states that he apparently only used a few minutes. That is not research and he didn't need to, he had all the relevant information on his company notebook anyway.
It's also a standing arrangement that the first sector a CC works they spend at least the take off and landing on the jump seat.
We do it during their ACM flight, but yes, that is normal. However, there are two pilots present and at the controls, still quite a chance for some mischief.
The new rule is unnecessary for those airlines that have phase 2 doors, i haven't flown for one that didn't. That they now use the two crew rule is simply to show the public that they do something. It doesn't increase safety, at best it leaves it at the same point where it was, more probably it lowers it.
Especially in those companies that just use non background/medical checked zero hour contractors in the cabin and have a high turnover.