Green Tiles, or Mirror, Mirror
Please keep these moments in mind - two green-tiled walls, one from HLV, one from TLD. But this is not the only parallel. Not by far.
In both scenes we get a very angry man - John Watson. Both times his aggression and frustration are closely tied to Sherlock. However, there is a clear progression from the situation in HLV - sexual frustration, dreaming of Sherlock, missing him, being disappointed to see it is not him in front of the door - to TLD - Mary is dead, John is a single father who has isolated himself and cut all ties with Sherlock. And it does not stop here.
The attacks: two junkies called Billy/William drawing sharp instruments (knife/scalpel) in the presence of John Watson:
Again, very similar situations but a clear progression where drama and heartbreak are concerned.
The fights: John Watson trying to disarm a junkie called Billy/William:
Again, similar but with a distinct progression towards violence in TLD.
The victims: two junkies called Billy/William, both physically hurt by John Watson
Once again, two mirrored scenes and once again the second is a progression of the first. The junkie now is Sherlock, the sprained wrist becomes a bloody face and possible injuries to the torso, the submissive attitude of the victim becomes much stronger.
And there is more. In HLV we have Billy as a Sherlock mirror. Whereas in TLD we have John plus a John mirror, i.e. Culverton Smith. TLD is John attacking Sherlock who is attacking a John mirror aka himself.
Summary: we have two situations which are similar but the second of which is an amplified version of the first in all regards: drama, violence, hurt.
So this is either a totally unrecognisable John Watson in TLD - which has been discussed extensively - or this is Sherlock creating a highly dramatised version of something that happened in HLV. True, he was not present when John disarmed Billy, but he knew that Billy had been hurt and must have assumed that a junkie/dealer would be armed in some way. Maybe he even knew the knife.
Now is there any evidence that the TLD scenes are not real? Well, Sherlock knows the drug den, he possibly has been there more than once. So he would know about the green tiles, right? And then we get this mysterious tiled room in TLD and we have no idea if this is in the hospital or at NSY or wherever else. And we get this very surreal conversation between John and Greg with John casually talking about the super secret fact of Sherlock having shot Magnussen in the face. Something you would probably tell either a friend nor an officer from Scotland Yard. The room, however, has these quite conspicuous green tiles and I would venture to say that this might be evidence of Sherlock re-creating an event from another place - the drug den - that is connected with his addictions - drugs and John Watson.
As for the presence of Culverton Smith: The fact that there are two people - John and a John mirror - who attack Sherlock in this scene is quite important. Sherlock is feeling trapped. He cannot trust his senses anymore. In a way this moment is also a reflection on the fireplace scene in THoB, another instance of him realising that everything he has relied on - his senses, John - is going to betray and abandon him.
I am sure you have lots of things to add so fire away!