Carrie Brown : Murder In The East River Hotel

Does the functionality of the door lock to Room 31 present more of a problem more than the key in terms of a solution ?

It isn't etched in stone that being in possession of the key automatically made C. Kniclo/Danish Farmhand the murderer.

sure, in a way it does.
The key in Frank’s possession is an easy solution that he was the killer - IF the door could not lock without the key.
Not necessarily correct solution because we can still come up with scenarios where Frank simply locks the door without being the killer.

I believe the door could not lock without key.

However even if it could, there’s still: Glenmore man, Train man, Frank being in out late and sullen the day after, the bloody shirt - and his proven (by the key) presence not only at the hotel but in the very room in which the murder occurred. So he’d still be implicated and look like a decent suspect.

The conundrum is still that Ali had to have been in the room after the murder before the door was locked but without the necessary (necessary at least in my opinion) key to lock it.