The locked door approach seems to have contributed to a number of incidents. Some form of key kept inside the flight deck that a crew member could take with them if they are leaving is something to look at. We seem happy to have a crash axe inside the flight deck, why not a key to get in the door as well.
Regarding why crew on a sub-2 hour flight even need to use The Facilities rather than before starting, that is I am afraid a feature of LCCs, minimum turnrounds at the outer point all scheduled by operational research down to the minute, everyone busy, and a desire to stick any delay in turnround on somebody. So the practice develops of doing so once you reach the cruise. Likewise the LCC justification for not having a cabin crew member step in is, I suspect, that on a sub-2 hour sector they are expected to be up and selling from the cart the moment the belts sign goes off. It's a question for the various boards of directors, whether commercial zeal has been allowed to get ahead of operational integrity.