Speculating a bit…
Yuukoku no Moriarty
I think, just a thesis, that it was between that case on the train and Irene Adler that William decided he wants Sherlock to be part – key figure – of his plan
Because prior to that, the case in which he tests Sherlock, he was offering Sherlock Holmes an in
What William offered him, information on who the great criminal is, was an invitation. If Sherlock Holmes had shot that man, like William set it up, then there were only two possible outcomes
Sherlock Holmes would join the Moriarty family – the Moriarty Pact
The Moriartys would need to “liquidate” Sherlock Holmes
It wasn’t just a test of Sherlock Holmes’ character, in the very same breath, it was an open invitation.
So, I think, prior to the murder on the train, William wagered with the idea of Sherlock Holmes joining their cause
He was still scrutinising Sherlock, still gauging who he is as a man, during that train case
There had to be one distinct point at which William decided how to proceed about Sherlock Holmes. The more likely one being the case around Adler, although it is possible that first ideas formed and solidified around their interaction on the train.
EDIT: in the manga canon it’s clear William already had his sights on Sherlock for his later role when he tested him with “A Study in ‘SH’”. This obviously influences what I said but I still think his repeated offerings towards Sherlock can be seen as William wanting him in the plan either way. Either as his chosen nemesis, his curated antagonist, or as part of the Moriarty Pact, in an active role Sherlock would be made aware of.
Because previously, with that chance of gaining Moriarty’s name. That offer of Sherlock committing one murder in exchange for such a valuable information, that would only leave Sherlock to join the Moriarty Pact or die.
But once the Irene Adler case happened, those documents affirmed the Moriarty Plan. Those documents solidified what the Moriartys had planned and it gave them a lead.
A red thread, if you will.
After the Irene Adler case, that was the point when William also locked Sherlock’s role into place. When he decided their whole plan would hinge on Sherlock Holmes.
Once their plan locked into place – because it was already set it motion and only gained a more determined shape through those documents – William had to have decided that he wanted Sherlock Holmes on the other side of it.
I assume William’s decision started to form earlier, someplace between that case on the train and Adler’s case.
EDIT: obviously between “A study in ‘SH’” and Adler’s case, considering where the manga draws attention to William having locked his sights on Sherlock
But then again there is the fact that even then, even as the case around Irene Adler set things into motion, William took a wager.
Moriarty’s name. The master criminal’s name. The true name of the Napoleon of Crime.
Quite literally handing that to Sherlock Holmes, full-well knowing Irene Adler would die… I think that was a final test.
William took a bargain, the chance of Sherlock opening that letter when he heard of Adler’s death was not zero. It wasn’t likely, for Sherlock to snap and open the letter, but there was no insurance. The chance never was zero
I think that was the final olive branch. Like how they showed Adler what James Moriarty is capable of, that letter was a final invitation extended to Sherlock Holmes.
Holmes refusing, burning that letter, it was what sealed the plan.
Sherlock Holmes himself sealed himself into that role William Moriarty wrote for him. That role William tailor-made for Sherlock Holmes, and Sherlock Holmes alone, to fulfil.
Obviously, later on, Sherlock defied expectations any way. And thoroughly so. But without knowing it, he made himself one of the key components of the Moriarty Plan.
And this is entirely besides the point of how formidably they played Mycroft Holmes – twice!
Because Mycroft– That case which enabled the MI6 to be founded, that kidnapping of William, Mycroft must have seen through the scheme.
At least the aftermath, the bigger picture of Moriarty orchestrating that kidnapping must be obvious to Mycroft.
Of course, to him it is Albert who pulled the strings. Towards Mycroft, it is Albert who orchestrated that Albert’s troop would be in London, that his brother William would be kidnapped, and that William would be taken to the lair of the opium smugglers.
That plan – because Mycroft Holmes must have seen through it for what it was – proved to Mycroft what Moriarty is capable of.
It doesn’t matter that, towards Mycroft, Albert was the mastermind. What mattered is that Albert showed him “this is what I can do with my resources I have now” and essentially left Mycroft no other choice but to hand the MI6 to Albert Moriarty.
William’s whole kidnapping, the entire set-up of it, was to show that Albert Moriarty knows how to use resources he has. That he knows how to orchestrate things.
That James Moriarty is fully capable of seeing through things others wouldn’t dream of attempting.
Of course, that is probably also where they earned Mycroft’s distrust because in proving how powerful, how organised, how capable the Moriartys are, they made him cautious about them.
They have been playing psychological games with Mycroft Holmes from the beginning. From the moment Albert got wind of the MI6 possibly being formed.
And while Mycroft was left with doubts about Albert’s loyalty, there were no doubts whatsoever about his capability.
The further you get into the story, the more it all falls into place, really.
