#1 One can ask, if this is in sherlock's dream/mind-palace why the episode starts with Watson's narrating voice? The reason is, while boarding already Sherlock was highly sedated and then he was going through Watson's stories (how he met with Watson in the first place and on), that's why in the dream it starts as Watson narrating the dream/Bride story.
#2 To differentiate between the DREAM and REAL there is a way-kept in the episode with calling names. In reality/regular life Sherlock calls Watson naming "John" and Watson calls naming "Sherlock" but in the dream they call each other with their last names. Sherlock calls Watson as "Watson" and Watson names Sherlock as "Holmes" this is the same reason Sherlock named Molly Hooper as "Hooper".
So, first we need to go 100 years back, and for that we need a highly sedative which Sherlock already had while boarding the plane. When the plane landed Sherlock handed over Mycroft a "list" about what are the sedatives Sherlock inhaled. (about the list the thing is, Mycroft and Sherlock had an agreement since childhood that whatever the sedative or whatever the dose, Sherlock will always keep a list of them while inhaling so that, if Sherlock goes fully senseless he can be recovered with the help of that list)
#3 Sherlock did not have much interest on the "Bride" story case. Only he need to know the connection between this bride story and Moriarty reborn. Another thing is important here, before the plane landing the full and everything is happening in sherlock's dream. So that means, anyone is saying anything is actually sherlock's idea.. cause the dream is in sherlock's mind. In one line, in this sherlock's dream Sherlock himself is both the script writer and director as well. So all the characters speech are actually sherlock's written script.
In fact Sherlock did solve the Bride story in the very very first place, when Watson says about the "secret twins" hearing this immediately Sherlock solved the case in a flash but wanted to go deep analysis to know every corner of it. That is why Mycroft could quote Sherlock "We don't defeat them, We most certainly lose to them" Mycroft could say this because Sherlock himself let Mycroft to say this (since this is Sherlock's dream).. that proves, Sherlock already did know the solve for Abominable Bride story.
Though the episode kept moving with the bride cases, but Sherlock tries to match the scenario with Moriarty in every possible aspect. That's why in the morgue, Sherlock think-fully one-time says "How could he survive?"
Bride story gets completely solved when Eustace gets murdered. cause without Lady Carmichael there was no else one to kill Eustace. In the Eustace murder actually there was no death note kept with the dagger. If there was any death note Sherlock definitely would see that at night when he found Eustace dead-body at first. There was not any death note but Sherlock's brain made up so and brings Moriarty with the same plot (more like sync the two story here). Why Sherlock brain did so? Cause Moriarty had already did same thing keeping clue in murder place in the earlier episodes, so maybe brain used that details here to sync the two stories.
#4 If we continue with Deep Dream idea, then 100 years back story is LEVEL- 2 deep dream. To wake up from LEVEL-2 it need to be a massive kick/wakeup call. So shaking is the #1st_Kick comes here when the plane lands, then Sherlock comes into reality which we can consider LEVEL-0. Then Marry, Mycroft, Watson was there in the plane (LEVEL-0) and immediately Sherlock brain updates its data with these Marry, Mycroft, Watson presence details (which brain can/may use ahead if needed*)
So to know the ending/more details Sherlock need to go back in his 100 years back story (LEVEL-2), but how could he?! Still Sherlock pushes himself for dreaming 100 years back (LEVEL-2) but since the sedative he took almost passed enough time and he is not in any calm environment anymore the dream becomes a bit of unstable situation (this is why while riding horse-cab, the mustache Watson and non-mustache Watson face was fluctuating).
[[[ Watson : Sherlock, tell me where my bloody wife is, you pompous prick ]]]
While explaining every detail to the hidden army, again Sherlock brain did sync Moriarty face with Carmichael..cause Moriarty and Carmichael are two main villains of two story.
When Moriarty says this is a dream going on (#2nd_Kick), the dream jumps down to LEVEL-1. This can be compared with light sleep dream (more closer to the realistic LEVEL-0, the dream seems real). This time Sherlock brain uses those data* picturing they are in graveyard. Brain uses details (Marry checking MI5) to find out where the Emelia dead- body was kept in rest. Since, this is LEVEL-1 everything looks so realistic and Sherlock even calls Watson naming "John" here (again I repeat, cause this is LEVEL-1)!
[[[ Watson : Since when do you call me John? ]]]
When Emelia dead-body jumps on Sherlock (this is the #3rd_Kick), the dream changes its LEVEL to LEVEL-2 again, which is in the Raichenbach falls (the water falls was shown in Mycroft room painting).
This time Sherlock is in LEVEL-2, but he knows he is LEVEL-2 deep now, mean he is in conscious now as well. For this reason, Sherlock calls Watson naming "John" here also. In one scene Watson also ask that ''since when do you call me John?" which proves assumptions okay.
Lastly, Sherlock needs a massive wake-up call to wake from LEVEL-2, this time it works as the waterfall jump (#4th_Kick).
#2 To differentiate between the DREAM and REAL there is a way-kept in the episode with calling names. In reality/regular life Sherlock calls Watson naming "John" and Watson calls naming "Sherlock" but in the dream they call each other with their last names. Sherlock calls Watson as "Watson" and Watson names Sherlock as "Holmes" this is the same reason Sherlock named Molly Hooper as "Hooper".
So, first we need to go 100 years back, and for that we need a highly sedative which Sherlock already had while boarding the plane. When the plane landed Sherlock handed over Mycroft a "list" about what are the sedatives Sherlock inhaled. (about the list the thing is, Mycroft and Sherlock had an agreement since childhood that whatever the sedative or whatever the dose, Sherlock will always keep a list of them while inhaling so that, if Sherlock goes fully senseless he can be recovered with the help of that list)
#3 Sherlock did not have much interest on the "Bride" story case. Only he need to know the connection between this bride story and Moriarty reborn. Another thing is important here, before the plane landing the full and everything is happening in sherlock's dream. So that means, anyone is saying anything is actually sherlock's idea.. cause the dream is in sherlock's mind. In one line, in this sherlock's dream Sherlock himself is both the script writer and director as well. So all the characters speech are actually sherlock's written script.
In fact Sherlock did solve the Bride story in the very very first place, when Watson says about the "secret twins" hearing this immediately Sherlock solved the case in a flash but wanted to go deep analysis to know every corner of it. That is why Mycroft could quote Sherlock "We don't defeat them, We most certainly lose to them" Mycroft could say this because Sherlock himself let Mycroft to say this (since this is Sherlock's dream).. that proves, Sherlock already did know the solve for Abominable Bride story.
Though the episode kept moving with the bride cases, but Sherlock tries to match the scenario with Moriarty in every possible aspect. That's why in the morgue, Sherlock think-fully one-time says "How could he survive?"
Bride story gets completely solved when Eustace gets murdered. cause without Lady Carmichael there was no else one to kill Eustace. In the Eustace murder actually there was no death note kept with the dagger. If there was any death note Sherlock definitely would see that at night when he found Eustace dead-body at first. There was not any death note but Sherlock's brain made up so and brings Moriarty with the same plot (more like sync the two story here). Why Sherlock brain did so? Cause Moriarty had already did same thing keeping clue in murder place in the earlier episodes, so maybe brain used that details here to sync the two stories.
#4 If we continue with Deep Dream idea, then 100 years back story is LEVEL- 2 deep dream. To wake up from LEVEL-2 it need to be a massive kick/wakeup call. So shaking is the #1st_Kick comes here when the plane lands, then Sherlock comes into reality which we can consider LEVEL-0. Then Marry, Mycroft, Watson was there in the plane (LEVEL-0) and immediately Sherlock brain updates its data with these Marry, Mycroft, Watson presence details (which brain can/may use ahead if needed*)
So to know the ending/more details Sherlock need to go back in his 100 years back story (LEVEL-2), but how could he?! Still Sherlock pushes himself for dreaming 100 years back (LEVEL-2) but since the sedative he took almost passed enough time and he is not in any calm environment anymore the dream becomes a bit of unstable situation (this is why while riding horse-cab, the mustache Watson and non-mustache Watson face was fluctuating).
[[[ Watson : Sherlock, tell me where my bloody wife is, you pompous prick ]]]
While explaining every detail to the hidden army, again Sherlock brain did sync Moriarty face with Carmichael..cause Moriarty and Carmichael are two main villains of two story.
When Moriarty says this is a dream going on (#2nd_Kick), the dream jumps down to LEVEL-1. This can be compared with light sleep dream (more closer to the realistic LEVEL-0, the dream seems real). This time Sherlock brain uses those data* picturing they are in graveyard. Brain uses details (Marry checking MI5) to find out where the Emelia dead- body was kept in rest. Since, this is LEVEL-1 everything looks so realistic and Sherlock even calls Watson naming "John" here (again I repeat, cause this is LEVEL-1)!
[[[ Watson : Since when do you call me John? ]]]
When Emelia dead-body jumps on Sherlock (this is the #3rd_Kick), the dream changes its LEVEL to LEVEL-2 again, which is in the Raichenbach falls (the water falls was shown in Mycroft room painting).
This time Sherlock is in LEVEL-2, but he knows he is LEVEL-2 deep now, mean he is in conscious now as well. For this reason, Sherlock calls Watson naming "John" here also. In one scene Watson also ask that ''since when do you call me John?" which proves assumptions okay.
Lastly, Sherlock needs a massive wake-up call to wake from LEVEL-2, this time it works as the waterfall jump (#4th_Kick).