Now then, let us begin going through these!
Watson is Holmes's drug.
I'm... not sure that particular phrase is really supported by the quote, although I can see the connection you're drawing. More relevantly tho, that whole paragraph is really just an explicit description of their entire relationship up to that point? I could probably cite several stories from each collection with supporting examples demonstrating it.
The statement of Holmes having become accustomed to and expecting Watson's presence could be claimed as a potentially unique point, but Watson has such a long history of stating that he or Holmes are accustomed to and easily identify each other's smallest habits that it's really more like a late entry into a consistent ongoing trend. I'm not entirely sure he doesn't state it earlier, even, but there are so many examples to go through it would take hours.
* * * * *
Watson has a "not inconsiderable" medical practice.
While the specific phrasing is not used earlier, the progression and success of Watson's medical practice is established extremely early! The information that he began medical practice again is established in
A Scandal in Bohemia, which was the very first short story ever published. The success of the practice is established in various other stories, most explicitly within
The Stockbroker's Clerk and
The Engineer's Thumb - both of which are in the
Memoirs.
A Scandal in Bohemia wrote:
One night—it was on the twentieth of March, 1888
[...]
"[...] I must be dull, indeed, if I do not pronounce him to be an active member of the medical profession."
The Stockbroker's Clerk wrote:
Shortly after my marriage I had bought a connection in the Paddington district. Old Mr. Farquhar, from whom I purchased it, had at one time an excellent general practice; but his age, and an affliction of the nature of St. Vitus’s dance from which he suffered, had very much thinned it. [...] I had confidence, however, in my own youth and energy, and was convinced that in a very few years the concern would be as flourishing as ever.
[...]
“Ah! Then you got hold of the best of the two.”
“I think I did. But how do you know?”
“By the steps, my boy. Yours are worn three inches deeper than his.”
The Adventure of the Engineer's Thumb wrote:
It was in the summer of ’89 [...] My practice had steadily increased, and as I happened to live at no very great distance from Paddington Station, I got a few patients from among the officials. One of these, whom I had cured of a painful and lingering disease, was never weary of advertising my virtues and of endeavouring to send me on every sufferer over whom he might have any influence.
* * * * *
Holmes has hired a person named Mercer
This one is a chronological and situational detail, rather than a character trait, so it doesn't qualify. I'm idly curious if Mercer shows up in any other prior stories, though, so I may look into it at some point. (The question is complicated by the fact that Doyle-as-Watson often refers to recurring incidental characters by role instead of name, so I'd have to actually read for it.)
* * * * *
Holmes thinks people should not try to transcend "Nature" and believes in destiny?
Ahh, I thought you were talking about a different story at first, but I've found the source and, since it's the same passage as the spoiler'd bit, I'm assuming this is about the same thing.
So, I think to address this fairly would involve a lengthy analytical discussion on what exactly Holmes
means by his idle philosophizing in that passage, which would involve drawing on past statements and behavior by Holmes as well as the language of the time and Doyle's own history and spiritualist beliefs, so while it is to me a potentially interesting topic, I don't think it's quite within the scope of this challenge and as such I'm going to leave it.