|Giving John time to see the Sun.|
Sherlock will immediately notice international professional assassins who move into the neighborhood "twenty feet from the front door." It's quite likely Sherlock told Mycroft about them, perhaps needing his help to identify them, or at least get more information than he has. There is also the possibility, with mobile phone communications being so easily monitored, that Sherlock and Mycroft are keeping any direct contact between themselves to a minimum, and really do have to send messages through John Watson, who can't be told he is serving as the telegraph line.
In any case, Mycroft confirms that John doesn't make the "rich brook/reichenbach" connection, but does know about the killers. Sherlock can't be the one to tell John or he blows his carefully cultivated for James Moriarty's benefit ordinary person facade. Like an actor who is always "on," Sherlock is spending 24 hours a day in character in order to convince the almost preternaturally intuitive Moriarty that he is still relatively clueless and believes the master criminal was only advertising his wares at the trial.
"He who wants to be a sickle must bend himself betimes."
from The Youth Who went Forth to Learn What Fear Was - Grimms' Fairytales
We know Sherlock finds a surveillance camera at the top of the built-in bookshelf behind him. We don't know who is watching or when it was placed there. We don't know how many there are. When James shows up at 221B, Sherlock doesn't engage in any witty repartee as he did by the pool, apparently moments from death. Here, in the relative safety of his flat, Sherlock gives away little and gathers in as much as possible. He asks the critical question, "What is it all for?" And Moriarty answers him truthfully, "I want to solve the problem. Our problem. The final problem."
James was wrong when he said he and Sherlock are "just alike." He doesn't know they no longer share the Final Problem of life's unbearable tedium. Sherlock changed. Sherlock has friends he cares for, who care for him. He has a future to look forward to, a brother reconciled with. And James isn't the only one with revenge on his mind. Sherlock has Carl Powers, an old blind woman and 11 other people who deserve justice. But James doesn't know about the change. And James, himself, cannot be released, solve his problem of staying alive until he pays Sherlock what he thinks he owes him, until Sherlock is obliterated.
The Reichenbach Fall 4