Those Senior and trained Cabin Crew with a long track record in the company should be the ONLY Cabin crew qualified and allowed to enter the Flightdeck
to allow any Cabin Crew to enter the cockpit with one flight crew member is asking for future trouble
That will not stop a determined sleeper. Look at the guys who planned 9/11. They were in it for the long game.
Then you also add the dimension of increasing the already steep authority gradient present in some cultures/airlines whereby a junior cc feels unable to contradict/intervene in a situation where a senior is showing poor judgement/unable to perform said duty... again... the solution creating its own problems...
737, I agree, that sort of environment won't help either, but there needs to be some sort of go between... even some kind of confidential help line for the industry where people can vent, get advice or even run the 'my boyfriend said XYZ, he's flying tomorrow, should I be worried?' type scenarios without fear of getting anyone sacked. Then they could follow up if they so chose...
Such a helpline exists in Australia for kids/young adults facing problems they feel they can't go to anyone else about. It's funded I believe by donations/govt funding. A few cents per airline ticket worldwide? Worth it.