Not all humans are from Earth in TOS. Dr. Miranda Jones, for instance, said in "Is There in Truth No Beauty?" that she had never been to Earth.
I suggest that Miranda is a Betazoid. For one thing it explains her telepathy without confirming that, in Star Trek, humans can do that. Deanna does get called "human," in a couple TNG episodes. In the Star Trek video games there is a Betazoid medic who had to study on Vulcan. It adds a whole new level of depth to the episode.
in order for the audience to even slightly believe that Scotty may have committed the crimes subconsciously
If Star Trek is treated as a continuous universe with episodes that could all happen in some order, then there is no way I could accept Scotty would do that.

just a way to get in some shore leave with a half hearted medical excuse
While it does sound like McCoy is suggesting Scotty has some real trauma, I think they must have had some further reason to attend that location, as I don't see taking an actual injured person with possible severe head trauma and also perhaps personality changes to that kind of establishment, or any kind of establishment, soon after, would be very wise.
But the only unanswered question is who was the woman and how did she cause the accident?
In stardate order the prior episode is "The Changeling." Due to Uhura's singing, Nomad tries read her brain which causes Scotty to react and Scotty intervenes and is "killed" but revived by Nomad. He does go flying across the room and may have hit his head.
The woman in question could be Uhura.
Kirk and McCoy are extra afraid for Scotty because they don't really know what Nomad did to him, and Scotty did go some time without fresh oxygen to the brain.
So they have reason to worry that Scotty has personality changes or other effects that they cannot quite predict. Kirk might keep calling it "a blow to the head" because saying "'killed' and revived" every time is almost to weired to accept in the first place.
It all fits together nicely
