You know what, I always thought Adam Greaves-Neal was probably one of the boys who auditioned for the part of young Sherlock - and perhaps they liked him but he was too young for the part or something - so they gave him another part. But I do like the meta giving a deeper meaning to his mirroring of Sherlock

Interesting Tumblr post, I'm not sure how strong of an argument it is but it's an interesting thought.  I don't know about it being two different parts of Sherlock (I don't understand the bit about Archie being more emotional), but the part of one being associated with different people and experiences are interesting.
Since the young Sherlock we see in Hist Last Vow is associated with Mycroft, and the episode before Sherlock tells Mycroft "I'm not a child anymore," that that little Sherlock represents something that Sherlock wants to put behind him, distance himself from.
On the other hand, he has an affinity for Archie, who is not connected to his past but is a lot like he would have been as a child.  And when he's advising Archie about how to perform at the wedding and telling Archie to wear the outfit, he tells Archie it's because "grownups" like that.  Archie asks why and Sherlock says "I don't know, I'll ask one." He's identifying himself as not being a "grownup," as if in some way he were also a child.  The child that exists through Archie still exists in him.

[Side note: Sherlock wears suits pretty much all the time.  Sure, they're not the same as the even more formal suits worn at the wedding but still..."grownups like that sort of thing." Is it a performance for Sherlock? Just spit-balling that one, as it came to mind as I was writing the rest.]

As for what you say, Liberty, I can kind of see how that could be, Steven's (and perhaps Mark's also) desire to have wanted to be around Sherlock Holmes to live through the experience of seeing his great mind solving a mystery.  I had never thought of that.

I like your thoughts about "the other one" being family, someone close. And I agree that "that day" may have a deeper meaning and will be revisited in the coming episodes. 

Louis looks a bit more like Steven than Archie does though .

Mixed thoughts on this.  I think seeing Archie as a young Sherlock is a bit of a stretch . I think it is meant to be a contrast with the way Mycroft treated Sherlock , and the way Sherlock treats Archie .
Sherlock understands and encourages and is kind to Archie , the way he wishes people did him ?
Sherlock in S01 seems to be a Sherlock following Mycrofts example ,all logic and denying sentiment ,  and yet also trying to break free of the influence because of a fall out.
Perhaps because of what happened to the other one .

Let me explain what I brought me to the Archie/Sherlock association. For me the scene in TGG in which Sherlock speaks about the death of Carl Powers is a key scene. We learn that Sherlock suspected a murder, went to the police, was not believed. Twenty years later Moriarty is back with a vengeance, the boy murderer has turned into a consulting criminal wreaking havoc on half the world, including Sherlock. 

Now take the Archie scene at the wedding banquet. Here an intelligent, maybe precocious little boy is believed, is taken seriously and because of this Sherlock manages to prevent a murder at John's wedding. Therefore I think Sherlock sees himself in Archie, the boy who had the right ideas but was not taken seriously by an adult world. 

 

What I meant was that Louis would have been more suitable to play a mini Steven Moffat!   But never mind, it was just a throwaway coment, obviously not as mildly amusing as I liked to think
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There's also the unseen boy in TGG, whom Sherlock feels an affinity with - into spies rather than detectives, but also could be an aspect of Sherlock's childhood.

SusiGo wrote:

Here are some more thoughts:

Sherlock/Archie/Jim - a long-term plan

Sorry but I cannot get this out of my head. Because there is an interesting connection between the Carl Powers case and the “invisible man with the invisible knife”. 

In TSoT Sherlock chooses to believe a child. He does not belittle or dismiss Archie’s idea but believes him and thereby manages to realise who is in danger. It is a decisive step in preventing Sholto from dying at John’s wedding. Sherlock may save the life by his deductions and John by his medical knowledge but it is Archie who sets it all in motion. 

Which reminds me of another boy who suspected that a murder had happened, went to the police, was not believed by the adult world. The murderer went free for the next twenty years, turning into a consulting criminal wreaking havoc on half the world. 

And then a similar but far more devastating thing happens - those in power, i.e. police and the media, again do not believe Sherlock but choose to believe that Sherlock himself is staging those crimes, not Moriarty.  

A lot of people made Jim Moriarty possible. 

Which, btw, reminds me of Mark’s recent words about the Moriarty arc:

“There’s a master plan. (…) There’s a plan. There is a long-term plan - very long-term.”

Indeed. Good spot Susi... also liking the idea of a very long term plan. ;)
 

Yes, well spotted, Susi.
A long-term plan. I like that.

 

I love the idea of The Plan.
But what is The Plan?!